The fuel indicator is unreliable...

[AuntPeggy]

I've just started instrument training (again) for the third time. Maybe this time is the charm.

Part of the introduction to instruments included the statement that I've heard many times from CFIs that the fuel gauge is only required to be accurate when it reads empty. It wasn't until I read the FARs for myself that I learned that it is also required to be relatively accurate throughout its range. No one told me that.

The upshot is that I have thought that the fuel indicator is completely unreliable and even felt guilty whenever I would check it to see whether there was sufficient fuel to continue flying. With the thought, "Well, it doesn't read 'empty' so there must be fuel in it," I continue on.

When I asked Hubby about it, he said that the statement is made that way so that pilots will do the fuel calculations. I have actually done the fuel calculations simply because I was told to. Silly me.

Anyway, in thinking about it, none of the instruments in my cockpit are required to be 100% accurate in any part of their range except the fuel indicator. This is not stressed either. OK, my lesson on Saturday was stressing it. We were taking a look at all the errors in the instruments. The heading indicator must be adjusted to the compass every 30 minutes or after you think it may have gotten screwy. The compass is inaccurate when climbing, descending, or turning. The altimeter is adjusted according to an ATIS or ASOS whenever the heading indicator is adjusted. The right wing in my turn coordinator rides a little high. The attitude indicator is also a little off. We all know there are a variety of ways the airspeed indicator is incorrect. The final invocation is, "BELIEVE IN YOUR INSTRUMENTS, NOT YOUR BODY."

Except the fuel indicator? Why is there such a stress on its inaccuracy that checking it is not taught? Which leads to the question of how often should it be checked? My guess is that it should be included in the half-hourly check of heading indicator and altimeter.

Come to think of it, there are a number of gauges over there on the right side or under the yoke that I haven't been taught how to use. How often should I be checking the voltage indicator, the OAT, the CHT, the suction gauge? What should I be checking for?

 

[Ted]

Fuel gauges are generally not particularly accurate, but they do tend to be consistent (at least, the ones in the planes I've flown are). An inaccurate fuel gauge will not cause you to invert the plane should you trust it as an inaccurate AI, DG/HSI, or TC might if you believed its erroneous reading. You've flown your plane enough at this point that you probably have a good idea of how it acts. So long as it acts the way you believe it's supposed to, you should (theoretically) be fine. I have only flown one plane that I would consider to have an "accurate" fuel gauge. The rest had varying levels of inaccuracy from "vague idea" to "completely out of the ballpark." You do the fuel calculations anyway, so that's a better idea. I just check mine to make sure they're doing what I expect them to. If they're not, there might be a problem, and I should keep a closer eye on them.

To the rest of the gauges you have:

- Voltage: Make sure this is within the proper range. The general thing to worry about here is if your alternator fails, your voltage will drop. Your avionics will then continue to work until your battery runs out of power. If your voltmeter is indicating lower than it's supposed to, get a backup plan going. If you're in a cloud, that backup plan should include getting out of the cloud, preferably on the ground.
- OAT: If this gets into the +3C or less range while you're in the clouds (or might get in the clouds) you could have icing concerns, and would therefore want to seek warmer air (and/or out of the clouds). Check this every few minutes.
- CHT: I don't know what sort of instrumentation you have. If it's an engine monitor, Lycoming has recommendations on what temperatures you should aim for. If it's just an analog CHT gauge, it's probably not very accurate. See that it's doing what it normally does and you should be fine. Check this every few minutes.
- Suction gauge: Make sure it's in the required operating range (this varies somewhat depending on plane). If it's not, then your vacuum instruments may not be working properly. Know which ones are your vacuum instruments. Check this every few minutes.

Airplanes generally work on steady state operation. So long as things are sitting where they're supposed to, things are likely just fine. I'd advocate a quick scan of the rest of the non-flight instruments every few minutes. You could incorporate it with your flight instrument scan, just remember that the first priority is keeping the plane in the appropriate orientation. Most of the other things can wait at least a little bit before action is required. If, say, your alternator dies, you do have some time before your electrical system goes out. So you have time to make sure you're not going to flip the plane upside down, and then call up the controller, explain the situation, and request direct to the nearest airport that you can get into (it does help if you have some idea of where this is).

Keep at it, you'll do fine.

 

[AuntPeggy]

Airplanes generally work on steady state operation. So long as things are sitting where they're supposed to, things are likely just fine. I'd advocate a quick scan of the rest of the non-flight instruments every few minutes. You could incorporate it with your flight instrument scan, just remember that the first priority is keeping the plane in the appropriate orientation. Most of the other things can wait at least a little bit before action is required. If, say, your alternator dies, you do have some time before your electrical system goes out. So you have time to make sure you're not going to flip the plane upside down, and then call up the controller, explain the situation, and request direct to the nearest airport that you can get into (it does help if you have some idea of where this is).

Keep at it, you'll do fine.

Thx. Is that a good rule of thumb for VFR pilots?

 

[Ted]

Thx. Is that a good rule of thumb for VFR pilots?

Absolutely. If your vacuum pump blows and your AI and DG/HSI go out, that can still cause confusion. Engines can still overheat in VFR conditions (although that's less of a concern in your 172), alternators can still go out, etc. Much easier to deal with if caught early.

For me, the primariy difference between VFR flight and IFR flight is how much I look out the window, although this is probably partly because I progress from private to instrument very quickly and use my instrument rating a lot. Obviously I'll also handle situations differently, but that's just being aware of your surroundings. If my electrical system fails in VFR (as it has) and I'm VFR to my destination, I'll just fly home with the master switch off. If I'm in IMC, I'm going to try to get on the ground before my battery dies. This is also part of why I keep my handheld GPS in the plane. It's not legal to shoot approaches with it, but it's a lot safer than just guessing when your electrical system goes out. Hopefully, I'll never have to do it for real, only for practice.

Ok, babbling off. Hopefully it provides you some useful info.

 

[RotorAndWing]

Part of the introduction to instruments included the statement that I've heard many times from CFIs that the fuel gauge is only required to be accurate when it reads empty. It wasn't until I read the FARs for myself that I learned that it is also required to be relatively accurate throughout its range. No one told me that.

People continuously try to apply FAR Part 23 standards to CAR3 airplanes. The Cessna 172 was certified under CAR3, not FAR 23.

CAR 3.761 Fuel quantity indicator. When the unusable fuel supply for any tank exceeds 1 gallon or 5 percent of the tank capacity, whichever is greater, a red band shall be placed on the indicator extending from the calibrated zero reading (see § 3.437) to the lowest reading obtainable in the level flight attitude, and a suitable notation in the Airplane Flight Manual shall be provided to indicate the flight personnel that the fuel remaining in the tank when the quantity indicator reaches zero cannot be used safely in flight.

3.672 Fuel quantity indicator. Means shall be provided to indicate to the flight personnel the quantity of fuel in each tank during flight. Tanks, the outlets and air spaces of which are interconnected, may be considered as one tank and need not be provided with separate indicators. Exposed sight gauges shall be so installed and guarded as to preclude the possibility of breakage or damage. Fuel quantity indicators shall be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply as defined by § 3.437.

 

[wabower]

My routine for gages is to try to be generally aware of all them by using a comprehensive scan, but I know that the little 2" gages on the far right aren't going to get as much attention as the primary flight instruments and avionics that are right under my nose. The beauty of analog gages is that their "Dr. Pepper" appearance is easy see at a glance.

Once established in cruise and the first full check completed, including fuel numbers recorded, I set a count-down timer for 20 minutes. When the time expires, I carefully check every instrument and setting in the cockpit. Fuel selector, circuit breakers, gages, knobs, levers, complete engine analyzer diagnostics, etc. When the check is complete, I reset the timer.

The 3x/hr routine seems to work well. Things don't seem to break often, but the continuity of the checks has been valuable on a few occasions. In addition to the (very) occasional problem, I've been able to catch a number of loose set screws, parts, screws on the floor that weren't there before, connectors, light bulbs, and other stuff.

If nothing else, the routine helps keep me awake and somewhat organized.

 

[Dan Thomas]

Except the fuel indicator? Why is there such a stress on its inaccuracy that checking it is not taught? Which leads to the question of how often should it be checked? My guess is that it should be included in the half-hourly check of heading indicator and altimeter.

Cessna issued a service bulletin in 1999 (SEB 99-18R1) that demands an annual accuracy check. Other manufacturers might have something similar. As far as accuracy goes, you can't expect precision from the technology still being used in these things.

In Canada, the Aircraft Requirements sections of the CARs lists the fuel gauge as a required item for flight in 605.14(j). It's not a deferrable item.

Dan

 

[mantakos]

I don't concern myself too much with what standards the fuel gauge was compared against for certification, because in the planes I fly, those fuel gauges are about 35 years old, and I've never heard anybody say "I had to put in a new fuel gauge, the old one tested to only 93% accurate".

If you fly the same plane consistently, then by repeatedly dipping the tanks in pre-flight and post-flight, and then comparing to the fuel gauge reading, you can, over time, get an idea of how your fuel gauge reads. In the planes I fly, each fuel gauge has its own peculiarities, and they certainly cannot be compared to each other, or compared to the markings on the gauge, but they are mostly consistent from flight to flight.

Ultimately, though, I really only use the fuel gauge as an indication that something has gone wrong. If it's indicating lower than I think it should be, that serves as my alarm that I may be leaking fuel in flight, and that I should get down on the ground and take a look. But they're just not accurate enough for me to say "well, I can see a little smidge of space between the needle and the E, so I'm good to go for another 15 minutes or so..."
-harry

 

[dmccormack]

I scan the fuel gauges every so often in cruise. Usually I’ll look that way more often on takeoff (if a cap is loose and fuel's gonna vent, it will happen then).

There are a few things I insist on, every single time, high wing, low wing -- even in my cork-float deal in the Chief -- look at the tanks and see how much fuel is there. No exceptions, no nuttin.

Once airborne, time tells me how much fuel I have. The only use I have for airplane fuel gauges is to let me know I've lost a bunch due to some other problem.

 

[RotorAndWing]

In any aircraft always have 2 methods to verify fuel quantity. When the 2 methods don't agree don't go until you have it figured out.

 

[gismo]

I've just started instrument training (again) for the third time. Maybe this time is the charm.

Part of the introduction to instruments included the statement that I've heard many times from CFIs that the fuel gauge is only required to be accurate when it reads empty. It wasn't until I read the FARs for myself that I learned that it is also required to be relatively accurate throughout its range. No one told me that.

The upshot is that I have thought that the fuel indicator is completely unreliable and even felt guilty whenever I would check it to see whether there was sufficient fuel to continue flying. With the thought, "Well, it doesn't read 'empty' so there must be fuel in it," I continue on.

When I asked Hubby about it, he said that the statement is made that way so that pilots will do the fuel calculations. I have actually done the fuel calculations simply because I was told to. Silly me.

Anyway, in thinking about it, none of the instruments in my cockpit are required to be 100% accurate in any part of their range except the fuel indicator. This is not stressed either. OK, my lesson on Saturday was stressing it. We were taking a look at all the errors in the instruments. The heading indicator must be adjusted to the compass every 30 minutes or after you think it may have gotten screwy. The compass is inaccurate when climbing, descending, or turning. The altimeter is adjusted according to an ATIS or ASOS whenever the heading indicator is adjusted. The right wing in my turn coordinator rides a little high. The attitude indicator is also a little off. We all know there are a variety of ways the airspeed indicator is incorrect. The final invocation is, "BELIEVE IN YOUR INSTRUMENTS, NOT YOUR BODY."

Except the fuel indicator? Why is there such a stress on its inaccuracy that checking it is not taught? Which leads to the question of how often should it be checked? My guess is that it should be included in the half-hourly check of heading indicator and altimeter.

Come to think of it, there are a number of gauges over there on the right side or under the yoke that I haven't been taught how to use. How often should I be checking the voltage indicator, the OAT, the CHT, the suction gauge? What should I be checking for?

First of all the notion that a fuel level indication system only needs to read "correctly" when the tank is empty is a myth. It's partially based on the fact that in most installations there is only one calibration adjustment that can be made and the maintenance procedures usually state that this adjustment should be done so as to make the gauge read empty when the tank is at zero usable fuel. But that doesn't mean that the certification requirements can be met by a fuel gauge that's only meaningful when the tank is empty, if that were true all it would take is a gauge that permanently reads zero.

Second, as others have said aircraft fuel gauges have historically been poorly designed and grossly inaccurate. Many will indicate something significantly different in a climb vs a descent and there are some airplanes with fuel gauges that are designed to read full until a substantial amount of fuel has been consumed. As a result it's common practice to teach students to never trust a fuel gauge, especially to the extent of assuming there's plenty of fuel in the tanks just because the gauges say so. I'm sure you were taught to gauge your fuel usage in terms of time in the air and the POH predicted fuel flow during climb and cruise and that's still a really good idea even if you "know" the gauges are pretty accurate in the plane you're flying. Another tool is a fuel "totalizer" which keeps track of the fuel consumed. They are generally very accurate (+/- 1% is typical) but will display erroneous data if there's a discrepancy between the actual fuel on board at the start of the flight and the initial amount set on the totalizer. They are also unable to recognize fuel lost to a misplaced cap or other leak since they only measure the fuel going into the engine.

There are more modern versions of fuel level sensing that are far more accurate than the old standby of a float tied to a variable resistor that most of our planes have.

I have a totalizer in my airplane and I use the fuel gauges (which don't work very well) to confirm the reading on the totalizer at various times on a flight. I compare them to the expected levels as part of the pre-taxi checks and re-confirm about once per hour, mostly to see that I'm not losing fuel somehow. If I were getting close to the bottom of the fuel supply per the totalizer I'd be checking the gauges more often. My plane has two tanks in each wing with separate gauges for each tank and when I want to run the aux tanks to near empty I'll check those levels often during the 10-15 minutes before switching back to the mains.

As to the other gauges, it's a good idea to check them about every ten scans or so but I have to admit that I'm pretty lax about that, partially because I have audible and visual warnings for all of them in addition to the gauges themselves (which are located way to the right and hard to include in a scan).

The most critical of those is the oil pressure. It's quite possible to have that drop to nothing at any time and outside of an indicator problem when that happens the prop will likely stop turning within a minute. The next most critical is probably the gauges for the electrical system. Most planes only have some sort of ammeter or loadmeter. An ammeter is the least useful because the normal indication (just above zero) is barely different than some abnormal indications (barely below zero), but if it shows a significant discharge (noticeably below zero) you will likely run out of electrical power within 10-15 minutes. A loadmeter is like an ammeter but it only reads above zero and shows the output of the charging system. As with the ammeter, it doesn't really show a minor problem very well and the "normal" reading depends mostly on the amount of equipment you have turned on. A voltmeter, OTOH, can tell you the state of the charging system at a glance. In a 12/14 volt airplane, anything below 13 volts means the battery is being discharged and anything above 14.5 means it's being overcharged along with potential damage to the avionics if it's really high. The normal range is 13.8 to 14.2 and as long as it's in that range everything is fine. For a 24/28 system those voltages are doubled. CHTs and oil temp are less critical but should be checked fairly often when climbing as that's the time you'll usually see higher than normal temps if something is wrong. You still should monitor them during cruise but unless your airplane is turbocharged or you are running really high power in cruise it's unlikely you'll see any problems then so a glance now and then should be sufficient.

 

[Jeanie]

Hi Aunt Peggy,

I have noticed re: the other gauges that I can see their indications fairly easily but I cannot read, with accuracy, the OAT. It's in the vent on the passenger side of my plane and I just can't see it well enough from my position on the left. So, I put a small red mark w/ a grease pencil on the 32 degree F. mark so that I know when the temp is freezing, and a yellow one on 40 deg F. That way I can glance up and have a quick read of the info.

I tried just placing the tube on my side but that didn't work at all and then I tried w/ the gauge in a position where straight down was 32 deg. but invariably it gets moved. So I marked it and it's been better.

With regard to the vacuum dying. My vacuum did die on my way into Atlanta. I was glad I was VFR. It was interesting to watch the AI slowly tumble and I can see how it would cause severe difficulty in IMC. IMO I would want to have some little stickies to cover up inop instruments to keep from the distraction it would cause.

Happy flying,
Jeanie

 

[jesse]

I consider the input from the fuel indicators to have some value. I don't FLY based on the indicators - meaning I know the fuel burn and how long I've been flying at a given power. I would never decide to fly further simply because the fuel gauge indicated more fuel then I thought I had.

BUT, if the fuel gauges indicated LOWER then I thought they should and lower then they have in the past for what I think the given burn is - you're damn right I'm going to take that into consideration. If they do something abnormal it might be because I made a mistake. I'd rather land and look into it.

In the airplane I fly IFR - I look at the oil pressure / voltmeter / and vaccum gauge about every two minutes. It really depends on load for me. If I haven't looked at them in awhile I'm aware of that and look. I look at the fuel gauges when I switch tanks or think about switching tanks. They aren't very accurate but I will take them into consideration if they're indicating something really abnormal.

 

[AuntPeggy]

Hi Aunt Peggy,

I have noticed re: the other gauges that I can see their indications fairly easily but I cannot read, with accuracy, the OAT. It's in the vent on the passenger side of my plane and I just can't see it well enough from my position on the left. So, I put a small red mark w/ a grease pencil on the 32 degree F. mark so that I know when the temp is freezing, and a yellow one on 40 deg F. That way I can glance up and have a quick read of the info.

I tried just placing the tube on my side but that didn't work at all and then I tried w/ the gauge in a position where straight down was 32 deg. but invariably it gets moved. So I marked it and it's been better.

With regard to the vacuum dying. My vacuum did die on my way into Atlanta. I was glad I was VFR. It was interesting to watch the AI slowly tumble and I can see how it would cause severe difficulty in IMC. IMO I would want to have some little stickies to cover up inop instruments to keep from the distraction it would cause.

Happy flying,
Jeanie

I have my OAT (C & F) on a gauge that also has the voltmeter - all in digital readouts - near the turn coordinator. Push a button to switch from one to the other. Guess I'm lucky there.

 

[AuntPeggy]

I consider the input from the fuel indicators to have some value. I don't FLY based on the indicators - meaning I know the fuel burn and how long I've been flying at a given power. I would never decide to fly further simply because the fuel gauge indicated more fuel then I thought I had.

BUT, if the fuel gauges indicated LOWER then I thought they should and lower then they have in the past for what I think the given burn is - you're damn right I'm going to take that into consideration. If they do something abnormal it might be because I made a mistake. I'd rather land and look into it.

In the airplane I fly IFR - I look at the oil pressure / voltmeter / and vaccum gauge about every two minutes. It really depends on load for me. If I haven't looked at them in awhile I'm aware of that and look. I look at the fuel gauges when I switch tanks or think about switching tanks. They aren't very accurate but I will take them into consideration if they're indicating something really abnormal.

Thanks.

I didn't mean to suggest I would fly further if it looked like I wasn't burning as much as expected. That would get me suspicious, too. But there are times when I'm wanting to skip an extra stop, feeling that super-safe would mean stopping, but I should be able to skip the extra stop if I watch the gauge to make sure my numbers were right.

 

[AuntPeggy]

First of all the notion that a fuel level indication system only needs to read "correctly" when the tank is empty is a myth. It's partially based on the fact that in most installations there is only one calibration adjustment that can be made and the maintenance procedures usually state that this adjustment should be done so as to make the gauge read empty when the tank is at zero usable fuel. But that doesn't mean that the certification requirements can be met by a fuel gauge that's only meaningful when the tank is empty, if that were true all it would take is a gauge that permanently reads zero.

Second, as others have said aircraft fuel gauges have historically been poorly designed and grossly inaccurate. Many will indicate something significantly different in a climb vs a descent and there are some airplanes with fuel gauges that are designed to read full until a substantial amount of fuel has been consumed. As a result it's common practice to teach students to never trust a fuel gauge, especially to the extent of assuming there's plenty of fuel in the tanks just because the gauges say so. I'm sure you were taught to gauge your fuel usage in terms of time in the air and the POH predicted fuel flow during climb and cruise and that's still a really good idea even if you "know" the gauges are pretty accurate in the plane you're flying. Another tool is a fuel "totalizer" which keeps track of the fuel consumed. They are generally very accurate (+/- 1% is typical) but will display erroneous data if there's a discrepancy between the actual fuel on board at the start of the flight and the initial amount set on the totalizer. They are also unable to recognize fuel lost to a misplaced cap or other leak since they only measure the fuel going into the engine.

There are more modern versions of fuel level sensing that are far more accurate than the old standby of a float tied to a variable resistor that most of our planes have.

I have a totalizer in my airplane and I use the fuel gauges (which don't work very well) to confirm the reading on the totalizer at various times on a flight. I compare them to the expected levels as part of the pre-taxi checks and re-confirm about once per hour, mostly to see that I'm not losing fuel somehow. If I were getting close to the bottom of the fuel supply per the totalizer I'd be checking the gauges more often. My plane has two tanks in each wing with separate gauges for each tank and when I want to run the aux tanks to near empty I'll check those levels often during the 10-15 minutes before switching back to the mains.

Thanks for the information. Hubby agrees. He says he checks on a scale. After an hour, then after 45 minutes, then 30 minutes, fifteen, ten, five minutes, then pretty much constantly. He is exaggerating, but the concept is there.

As to the other gauges, it's a good idea to check them about every ten scans or so but I have to admit that I'm pretty lax about that, partially because I have audible and visual warnings for all of them in addition to the gauges themselves (which are located way to the right and hard to include in a scan).

The most critical of those is the oil pressure. It's quite possible to have that drop to nothing at any time and outside of an indicator problem when that happens the prop will likely stop turning within a minute. The next most critical is probably the gauges for the electrical system. Most planes only have some sort of ammeter or loadmeter. An ammeter is the least useful because the normal indication (just above zero) is barely different than some abnormal indications (barely below zero), but if it shows a significant discharge (noticeably below zero) you will likely run out of electrical power within 10-15 minutes. A loadmeter is like an ammeter but it only reads above zero and shows the output of the charging system. As with the ammeter, it doesn't really show a minor problem very well and the "normal" reading depends mostly on the amount of equipment you have turned on. A voltmeter, OTOH, can tell you the state of the charging system at a glance. In a 12/14 volt airplane, anything below 13 volts means the battery is being discharged and anything above 14.5 means it's being overcharged along with potential damage to the avionics if it's really high. The normal range is 13.8 to 14.2 and as long as it's in that range everything is fine. For a 24/28 system those voltages are doubled. CHTs and oil temp are less critical but should be checked fairly often when climbing as that's the time you'll usually see higher than normal temps if something is wrong. You still should monitor them during cruise but unless your airplane is turbocharged or you are running really high power in cruise it's unlikely you'll see any problems then so a glance now and then should be sufficient.

We have an ammeter that I check to see if it is positive at runup and a voltmeter that stays about 14.2 except a couple of times when we had an electrical failure. It was the transponder turning itself off that told us of the electrical failure. I do monitor the CHT while climbing, but simply because I was told that it might be necessary to re-lean. A friend recently died when he got an oil problem and lost the engine. Since then, I've been looking at the oil temperature. Good to know when to watch it more carefully. My C-172 is definitely not turbo charged.

 

[AuntPeggy]

In any aircraft always have 2 methods to verify fuel quantity. When the 2 methods don't agree don't go until you have it figured out.

By that I assume you mean
1) the calculated burn.
2) the fuel gauge indication.

 

[AuntPeggy]

I scan the fuel gauges every so often in cruise. Usually I’ll look that way more often on takeoff (if a cap is loose and fuel's gonna vent, it will happen then).

There are a few things I insist on, every single time, high wing, low wing -- even in my cork-float deal in the Chief -- look at the tanks and see how much fuel is there. No exceptions, no nuttin.

Once airborne, time tells me how much fuel I have. The only use I have for airplane fuel gauges is to let me know I've lost a bunch due to some other problem.

We dip fuel, too, and write the numbers for fuel and oil in a logbook kept in the airplane. It makes us not only look, but remember how much fuel is in the tanks.

 

[jesse]

Since then, I've been looking at the oil temperature. Good to know when to watch it more carefully. My C-172 is definitely not turbo charged.

Keep in mind that the oil pressure is a better indicator of a MAJOR oil related issue than the temperature is.

 

[gismo]

With regard to the vacuum dying. My vacuum did die on my way into Atlanta. I was glad I was VFR. It was interesting to watch the AI slowly tumble and I can see how it would cause severe difficulty in IMC. IMO I would want to have some little stickies to cover up inop instruments to keep from the distraction it would cause.

Happy flying,
Jeanie

One of the "best buys" for an IFR airplane is a prominent warning light for low vacuum. Precise Flight sells one for about $80 and the installation shouldn't take more than a couple hours of shop time.

www.aircraftspruce.com/catalog/inpages/vacuumwarning.php

It's not all that difficult to fly a plane with "needle ball and airspeed" but it can be rather difficult to recover from an unusual attitude on partial panel especially when near the ground. One common scenario when the pump fails is for the horizon gyro to slowly (over a minute or so) roll over to one side with an increasing error in pitch as well. At about the same time the DG is likely to begin precessing (rotating) and it's been shown that pilots often will follow that horizon into a fairly steep bank before realizing that something is amiss. Having a bright red light next to the horizon that comes on when the pump fails (and a few minutes before the gyros start to die) can make the difference between attempting an upset recovery with partial panel and simply keeping the wings level while covering up the soon to be misleading Ai and DG. I strongly recommend anyone flying behind steam gauges to find a way to add this inexpensive device. IMO it's far more benificial than a backup vacuum pump.

 

[Clark1961]

One of the "best buys" for an IFR airplane is a prominent warning light for low vacuum. Precise Flight sells one for about $80 and the installation shouldn't take more than a couple hours of shop time.

www.aircraftspruce.com/catalog/inpages/vacuumwarning.php

It's not all that difficult to fly a plane with "needle ball and airspeed" but it can be rather difficult to recover from an unusual attitude on partial panel especially when near the ground. One common scenario when the pump fails is for the horizon gyro to slowly (over a minute or so) roll over to one side with an increasing error in pitch as well. At about the same time the DG is likely to begin precessing (rotating) and it's been shown that pilots often will follow that horizon into a fairly steep bank before realizing that something is amiss. Having a bright red light next to the horizon that comes on when the pump fails (and a few minutes before the gyros start to die) can make the difference between attempting an upset recovery with partial panel and simply keeping the wings level while covering up the soon to be misleading Ai and DG. I strongly recommend anyone flying behind steam gauges to find a way to add this inexpensive device. IMO it's far more benificial than a backup vacuum pump.

This is an interesting perspective. I was taught "Mickey Mouse" for unusual attitude recovery. That is to look at the DG, altimeter, and airspeed and apply appropriate controls (the DG, altimeter and airspeed make a mouse ears and head). The attitude indicator is just an afterthought for conformation. Of course if vacuum has failed then the DG is going to be wrong but at least the altimeter and airspeed will get you leveled off.

Of course we want to avoid any unusual attitude but it is nice to have been taught a fairly reliable method of recovery. Of course iced over Pitot and static ports will ruin the day...so keep the heat on.

 

[gismo]

This is an interesting perspective. I was taught "Mickey Mouse" for unusual attitude recovery. That is to look at the DG, altimeter, and airspeed and apply appropriate controls (the DG, altimeter and airspeed make a mouse ears and head). The attitude indicator is just an afterthought for conformation. Of course if vacuum has failed then the DG is going to be wrong but at least the altimeter and airspeed will get you leveled off.

Of course we want to avoid any unusual attitude but it is nice to have been taught a fairly reliable method of recovery. Of course iced over Pitot and static ports will ruin the day...so keep the heat on.

But wouldn't it be better to avoid the unusual attitude in the first place?

 

[Clark1961]

But wouldn't it be better to avoid the unusual attitude in the first place?

Ummm yup! I think I said that with the "avoid any unusual attitude" comment. Of course YMWV!

 

[gismo]

Ummm yup! I think I said that with the "avoid any unusual attitude" comment. Of course YMWV!

My bad. I interpreted the first paragraph to mean that you didn't see the point in adding an inexpensive vacuum loss indicator since it was feasible to recover from a failing gyro induced upset and completely missed your last sentence.

 

[RotorAndWing]

By that I assume you mean
1) the calculated burn.
2) the fuel gauge indication.

Keep a "fuel score", a written fuel log. Start with a known quantity (full) then log each time you fly and when you add fuel. Also create a dip stick or get one of those glass types. Compare the two observations.

This can also be done for aircraft with fuel totalizers.

 

[AuntPeggy]

Keep a "fuel score", a written fuel log. Start with a known quantity (full) then log each time you fly and when you add fuel. Also create a dip stick or get one of those glass types. Compare the two observations.

This can also be done for aircraft with fuel totalizers.

Oh, now I understand. I do keep that information in an Excel spreadsheet. Lastnight I looked at the average, 7.6 g/h for the year 2008. I haven't copied the airplane log and entered 2009 yet.

We keep a record of the dipstick for each tank in our airplane log, too.

Edit: Our airplane log is the informal one we keep in the aircraft where we keep track of flights.

 

[TMetzinger]

Keep a "fuel score", a written fuel log. Start with a known quantity (full) then log each time you fly and when you add fuel. Also create a dip stick or get one of those glass types. Compare the two observations.

This can also be done for aircraft with fuel totalizers.

Very good idea - fuel flow/totalizer systems can be miscalibrated and dreadfully wrong, too! We had the fuel flow sensor in the Mooney replaced and the tech didn't catch the notice that the replacement unit had a different "K" factor than the original unit... It was quite a surprise to see only 6 GPH on takeoff (when I should have seen 14+). If I hadn't had it set to GPH, I might have "believed" it over the fuel gauges and skipped a fuel stop someday, with bad results.

Once you've done the fuel measuring with a dipstick or glass tubes a few times on trips of varying times, you'll get a good feel for how much fuel you burn per hour on "average". That can help you spot discrepancies early enough to safely do something about them.

 

[rottydaddy]

I often disparage fuel indicators, but only because they are a very poor substitute for visually verifying or "sticking" the tanks and then knowing your fuel burn. It's a "useful prejudice", you might say.

I've flown quite a few different rented Cessnas, and found that the gauges were not really as bad as the scuttlebutt indicates, but generally at least one indicator was way off. Even the wonderfully simple "bobber and needle" style indicators found on some older types can lie to you if the float is in poor condition or what-have-you. The Champ I flew had a very simple float-and-drum indicator that jumped around so much it was virtually un-readable... a float with a wire protruding through the filler cap would be better.... but still not completely immune to error.


But in flight, all you have left is your estimated fuel burn and the gauges. If your estimate turned out to be wrong, or if there is a leak, the gauges may tell you about it, so they're worth checking from time to time. If you make a precautionary landing based on a bogus fuel indicator reading, that's annoying, but not as annoying as landing off-field with dry tanks because you were positive there was more fuel than indicated...


For my kind of flying, though,the best policy of all, other than visually verifying before flight, making sure you see no leaking fuel on the ramp or loose filler caps, and knowing your burn, is to not plan or execute legs that require most of the fuel on board.

 

[TMetzinger]

I often disparage fuel indicators, but only because they are a very poor substitute for visually verifying or "sticking" the tanks and then knowing your fuel burn. It's a "useful prejudice", you might say.

I've flown quite a few different rented Cessnas, and found that the gauges were not really as bad as the scuttlebutt indicates, but generally at least one indicator was way off. Even the wonderfully simple "bobber and needle" style indicators found on some older types can lie to you if the float is in poor condition or what-have-you. The Champ I flew had a very simple float-and-drum indicator that jumped around so much it was virtually un-readable... a float with a wire protruding through the filler cap would be better.... but still not completely immune to error.


But in flight, all you have left is your estimated fuel burn and the gauges. If your estimate turned out to be wrong, or if there is a leak, the gauges may tell you about it, so they're worth checking from time to time. If you make a precautionary landing based on a bogus fuel indicator reading, that's annoying, but not as annoying as landing off-field with dry tanks because you were positive there was more fuel than indicated...


For my kind of flying, though,the best policy of all, other than visually verifying before flight, making sure you see no leaking fuel on the ramp or loose filler caps, and knowing your burn, is to not plan or execute legs that require most of the fuel on board.

I recently built a scenario for the Redbird sim where a pilot is flying along in his G1000 182 with a fuel leak. We essentially "lost" 20 gallons of fuel overboard over the first hour of flight. This caused the gauges to be telling the truth but the fuel flow/totalizer to be lying to the pilot. The object of the scenario is for the pilot to catch the discrepancy between the gauges and the totalizer BEFORE the low-fuel warning from the gauges comes on.

The pilot scores highest if he detects the discrepancy early and diverts and lands to determine the cause. The earlier, the better
The pilot scores in the middle if he doesn't detect the discrepancy but responds to the low-fuel warning by diverting immediately to the nearest airport.
The pilot scores lowest if he ignores the low fuel level alert (believing the totalizer) and runs out of gas.

 

[Dan Thomas]

I recently built a scenario for the Redbird sim where a pilot is flying along in his G1000 182 with a fuel leak. We essentially "lost" 20 gallons of fuel overboard over the first hour of flight. This caused the gauges to be telling the truth but the fuel flow/totalizer to be lying to the pilot. The object of the scenario is for the pilot to catch the discrepancy between the gauges and the totalizer BEFORE the low-fuel warning from the gauges comes on.

There's at least one scenario where a massive fuel loss can occur, yet the fuel gauges will still read high. Here it is:

83-13-01 CESSNA: Amendment 39-4672. Applies to Models 182, 182A, 182B, 182C, 182D, 182E, 182F, 182G, 182H, 182J, 182K, 182L, 182M, 182N, 182P, 182Q (all serial numbers except 66590 and on) and R182 (S/N R18200002 through R18200583) airplanes certificated in any category.

Compliance: Required as indicated, unless already accomplished.

To alert the pilot to the potential effects of improper fuel cap sealing:

a) Within the next 12 calendar months after the effective date of this AD install a placard adjacent to the fuel quantity gauges which states: "CAUTION Leaking fuel caps can cause loss of fuel and erroneously high fuel quantity indications."

This placard may be fabricated by the owner/operator of the airplane. The person accomplishing this must make the prescribed entry in the aircraft maintenance records reflecting compliance with paragraph a) of this AD.

b) Within the next 12 calendar months after the effective date of this AD and each 12 calendar months thereafter:

1) Visually inspect, the surface of the wing aft of the fuel cap for evidence of leakage, the fuel cap seals for cracks, distortion and or any condition which may prevent sealing and the sealing surface of the adapter for scratches, corrosion, distortion or other conditions which may prevent sealing. If any of these conditions are noted inspect the fuel tank for wrinkles in the bottom and proper attachment of the retaining snaps to the compartment. Prior to further flight, correct any unsatisfactory conditions in accordance with the manufacturers maintenance manuals or service information which should include inspection of the fuel tank installation in accordance with Cessna Service Letter SE82-34A. Fuel cap repairs should be accomplished in accordance with Cessna Service Letter SE80-59 Supplement 1.

<snip>

It's a result of the fuel cap O-rings shrinking or breaking. These flush caps are in the low-pressure zone atop the wing, and if they leak enough the underwing fuel vent can't keep up with the vapor loss. The tanks, being bladders, start to collapse under the low pressure, and squeeze the fuel out past the leaky cap. Since all that suction is on top, the bladder is sucked upward and lifts the fuel sender float and make the gauge read high, maybe even full. And it could pull fuel across from the other tank, too.

Leaving the cap off a bladder tank would probably do the same thing.

Dan

 

[dmccormack]

There's at least one scenario where a massive fuel loss can occur, yet the fuel gauges will still read high. Here it is:

83-13-01 CESSNA: Amendment 39-4672. Applies to Models 182, 182A, 182B, 182C, 182D, 182E, 182F, 182G, 182H, 182J, 182K, 182L, 182M, 182N, 182P, 182Q (all serial numbers except 66590 and on) and R182 (S/N R18200002 through R18200583) airplanes certificated in any category.

Compliance: Required as indicated, unless already accomplished.

I'm surprised the C205 isn't listed, as it has the same fuel tank setup...

Or does it?

 

[TMetzinger]

<snip>

It's a result of the fuel cap O-rings shrinking or breaking. These flush caps are in the low-pressure zone atop the wing, and if they leak enough the underwing fuel vent can't keep up with the vapor loss. The tanks, being bladders, start to collapse under the low pressure, and squeeze the fuel out past the leaky cap. Since all that suction is on top, the bladder is sucked upward and lifts the fuel sender float and make the gauge read high, maybe even full. And it could pull fuel across from the other tank, too.

Leaving the cap off a bladder tank would probably do the same thing.

Dan

Very cool - thanks for sharing. I may use that scenario in the steam version sim. "Go fly for an hour and let's see if you notice your fuel gauges still read full. What do you do about it?"

 

[dmccormack]

Very cool - thanks for sharing. I may use that scenario in the steam version sim. "Go fly for an hour and let's see if you notice your fuel gauges still read full. What do you do about it?"

How would he/she know there's a problem in a sim? After all, all you have are the panel indications..

 

[TMetzinger]

How would he/she know there's a problem in a sim? After all, all you have are the panel indications..

The point of either exercise is to cross check what you expect with what you actually see.

In the first exercise - the fuel gauges will be indicating less fuel than the totalizer. Spot the discrepancy and land to figure it out is the desired outcome.

In the second exercise - you'll be flying for a while and expect (in a Cessna with the fuel selector on Both) to see both fuel gauges decreasing over time. When you DON'T see that (perhaps with one cap leaking you'll see one tank reading "full" while the other goes down at a more rapid than expected pace), you should land to figure it out.

It's all about developing the mental habit of scanning and thinking about everything, and not being complacent.

Make sense now?

 

[dmccormack]

The point of either exercise is to cross check what you expect with what you actually see.

In the first exercise - the fuel gauges will be indicating less fuel than the totalizer. Spot the discrepancy and land to figure it out is the desired outcome.

In the second exercise - you'll be flying for a while and expect (in a Cessna with the fuel selector on Both) to see both fuel gauges decreasing over time. When you DON'T see that (perhaps with one cap leaking you'll see one tank reading "full" while the other goes down at a more rapid than expected pace), you should land to figure it out.

It's all about developing the mental habit of scanning and thinking about everything, and not being complacent.

Make sense now?

Hmmm.....

I suppose. It makes sense to run that sort of scenario in a sim (unless you have some magic siphon for aerial defueling), but unless the pilot's familiar with the fuel system, it may be more a gotchya than a learning opportunity.



For example, in older Bonanzas, excess fuel bypasses the carb and is returned to the left tank. Thus left tank for an hour with full tanks makes sense. But after than don't be surprised to see the left tank quantity increase.

Confusing -- yep. But this is really an airplane-specific knowledge chunk that doesn't transfer well from one to another.

The C205 doesn't have a BOTH position, and despite being a high wing Cessna, has two fuel pumps (engine driven and electric). The electric pumps has a high and low setting. Hit HIGH in flight and say hello to a drowned engine.

Yet the POH recommends hitting the electric pump when switching tanks. Given the HIGH and LOW rockers are parallel, I don't turn on the pump -- and it works fine.

Anyway, I think the fuel system would have to be known and understood before someone would spot some vague difference in consumption, methinks.

 

[TMetzinger]

Yes... this is not to teach a primary student something. This is to teach a student who is already supposed to know how the fuel system in his airplane (Sim is a 182) works. The "magic" of the sim allows us to fast-forward through 50 minutes after takeoff, and present the student with the situation that's one hour into the flight but only 12 minutes in real time.

And the lesson to be learned is to pay attention to detail. Another scenario involves catching on to the fact that the headwinds are stronger than predicted before you end up running out of gas over the desert.

And hey, if you go flying in an airplane and you don't know how the fuel system works, there's a "gotcha" just waiting to eat your face. Better to learn that in a sim, no?

 

[dmccormack]

Yes... this is not to teach a primary student something. This is to teach a student who is already supposed to know how the fuel system in his airplane (Sim is a 182) works. The "magic" of the sim allows us to fast-forward through 50 minutes after takeoff, and present the student with the situation that's one hour into the flight but only 12 minutes in real time.

And the lesson to be learned is to pay attention to detail. Another scenario involves catching on to the fact that the headwinds are stronger than predicted before you end up running out of gas over the desert.

And hey, if you go flying in an airplane and you don't know how the fuel system works, there's a "gotcha" just waiting to eat your face. Better to learn that in a sim, no?

As long as the sim is set up to replicate a specific airplane, sure.



Can your sim replicate a header tank with cork and wire?

 

[TMetzinger]

As long as the sim is set up to replicate a specific airplane, sure.



Can your sim replicate a header tank with cork and wire?

Hmm... The thing is based on FS X, so it probably could... getting that past the FAA review would be interesting.

 

[dmccormack]

Hmm... The thing is based on FS X, so it probably could... getting that past the FAA review would be interesting.

I tried XPlane and finally decided to go back to FSX.

XPlane was just a resource hog.

 

[gismo]

It's a result of the fuel cap O-rings shrinking or breaking. These flush caps are in the low-pressure zone atop the wing, and if they leak enough the underwing fuel vent can't keep up with the vapor loss. The tanks, being bladders, start to collapse under the low pressure, and squeeze the fuel out past the leaky cap. Since all that suction is on top, the bladder is sucked upward and lifts the fuel sender float and make the gauge read high, maybe even full. And it could pull fuel across from the other tank, too.

I had a similar problem in a Bonanza (which has bladder tanks). The fuel vent had frozen over and the bladder got sucked up as the tank was emptied. Without considering how much fuel was supposed to be in the tank I switched to it on an approach in a snowstorm (part of the pre-landing checklist was "switch to the fullest tank" and that one read the highest). The engine quit just before I reached the final approach fix. I resolved the crisis by switching to a different tank and the engine started running again but not as fast as my heart was beating.