Identify MAP by Timing

Areeda

Areeda

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Areeda
Background: I'm doing oral prep for an insturment rating with another instructor's student for his retest.

I'm looking for a reference to how to determine the time from FAF to MAP.

I believe you estimate your ground speed and interpolate the timing table at the bottom of the approach plate. He says, you take the time from the next higher speed.

I've checked the books I have available (AIM, Instrument Flying Handbook, Instrument Procedures Handbook, Jeppesen Instrument/Commercial, PIC, Kirshner, Machado [can't find Gardner]) and I can't find anything on how to determine the time.

I tried to explain the error if you round 100 kts up to 120 can result in declaring missed early but all I get is "I was taught to round my speed up to the next value in the table".

A reference would really help. I have to admit, I have nothing authoritative to back my position either.

Thanks
Joe
 
Interpolation.

If I am going 95 kts, I am not going to use the 120kt reference. Especially when you have 5:00 at 90kts, ends up being 3:45 at 120 - but only 4:44 at 95kts. If you used the next higher on the table you'd be about 1.5 miles short of the real MAP.

Let's see, 1.5 miles short of the map, and the visibility is 1sm...

Student said:
"I was taught to round my speed up to the next value in the table".
Click to expand...
What Joe Areeda Should Say said:
I see why you need a retest.
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I'd agree with EdFred's logic...look at a couple of approaches and rounding errors that could be possible. It isn't necessarily an "error" to simply round up, IMO, but it can result in making successful completion of the approach impossible if misused.

Interpolation is an accepted technique for calculations in aviation. I doubt that any text will specifically talk about interpolation in this case, as it's probably considered "too basic"...kind of like using addition, subtraction, multiplication, and division to compute a weight and balance. Like many things in aviation, it's allowed unless it's specifically prohibited.

Similarly, using the time for the next higher groundspeed isn't specifically authorized. Nor, for that matter, is using a groundspeed that's not specifically noted on the chart. Technically, there is nothing that says you can shoot an approach with a groundspeed of 105 knots, right? ;)

Fly safe!

David
 
most of my students are busy enough flying the approach, and for most of them, mental math is something crazy people like me do. you should see some of the blank stares i get when we are doing ground on perf charts or w&b. "I get a calculator for the written, right?"
 
I don't think a reference exists. I looked.

Each of the three good CFII's I've used was okay with my interpolation, since I was flying about 100kt approaches. We never discussed this deeper. The one 'bad' CFII I used was okay with it too, but that's probably a notch in the wrong corner...
 
FAA Instrument Procedures Handbook said:
When a missed approach is executed
prior to reaching the MAP, the pilot is required to continue
along the final approach course, at an altitude
above the DA, DH, or MDA, until reaching the MAP
before making any turns. If a turn is initiated prior to
the MAP, obstacle clearance is not guaranteed.
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Time it "short" and go missed, you could turn into the side of a mountain or tower. Timing based on speed is critical to get as close to "right" as you can. Get the GS off your GPS or whiz-wheel, interpolate the charted times, fly the time.
 
I was taught to fly the approach at 90 knots IAS (which was appropriate for the airplane) because the published time to MAP was based on 90 knots (and 120, etc.). Groundspeed, while technically important, is in reality not worth doing higher mathematics to get precise. You don't have time to do that anyway.

Since you're looking for an altitude, not a location, for your MAP, wherever you are when the timer goes off will most likely be "close enough". Since you'll probably have a headwind and not a tailwind, you'll reach MAP early, so just level off you'll get there just fine.

That's not a one-size-fits-all solution, nor is it a perfect FAA-acceptable, Designated Examiner approved answer. :fcross:
 
JohnR said:
Groundspeed, while technically important, is in reality not worth doing higher mathematics to get precise. You don't have time to do that anyway.
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Except for that time I had 45kt headwinds at 3000' AGL. Air was smooth that day too. I'd have only been, oh, off by a factor of two. It IS worth doing.
 
thats an interesting document troy. standard 141 CYA kind of stuff. i find the section on the operation of their motorglider interesting. very restrictive.
 
I have never before heard of rounding up. The interpolation method is the only one I've ever heard of, used, or taught in 36 years of instrument flying. The reasons discussed above are sufficient for me to say the "rounding up" method would be inappropriate and potentially dangerous.
 
How about just adjusting your speed to fit an appropriate line on the approach plate? You have a throttle and flaps, and the plane will fly at a range of speeds with an appropriate rate of descent. You can always adjust your configuration a little when you pop out into visual for the short final. Works even with a 30kt headwind. If you usually fly the 120kt times, fly the 90kt times with air speed of 120kts. More complicated when it is not straight in, but isn't that the basic idea? :rolleyes: Not much "higher math" involved in adding 90+30 and setting the throttle accordingly, right? Or, if you usually fly the 90kt times, fly the 60kt times at an air speed of 90kts. For a 45kt headwind, I would be flying the 60kt times at 105kts airspeed. (60+45=105)
 
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My note was just to indicate the OP process starts at the wrong end, if you want simplicity. And simplicity helps a lot. Interpolation of time is more complicated than adjusting your air speed to fit the published numbers. If you have been flying into an area IFR you ought to have been making the IAS to CAS to TAS to GS adjustments all along to stay on your schedule, so should know what adjustment is needed for that. The time interpolation on the approach is the only thing new, and my point was it is unnecessary.

Off the subject a little, but the VFR GPS units are only good for "situational awareness" when IFR. One situation it can make you aware of is your actual ground speed. A good cross check to be sure you are not making a gross error about what timing to use.
 
Dwight B. Van Zanen said:
If you have been flying into an area IFR you ought to have been making the IAS to CAS to TAS to GS adjustments all along to stay on your schedule,
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Military bombers do that to make designated time on target (TOT), but that's an issue of coordinating attacks by multiple aircraft without biffing each other. However, I have never, ever heard of anyone in the civilian world adjusting speed to make flight plan ETA. That's a good way to throw a big monkey wrench in the controllers' works.
 
Boo Boo said:
How about being approved for an airspeed adjustment to arrive so as to minimize or eliminate a hold by arriving at the EFC time or maximize holding endurance due to reduced fuel burn en-route to the hold? Or do I need some letter from ATC saying that procedure is acceptabe?
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If you can find the procedure for that in the AIM or FAA Order 7110.65, please let me know -- I can't. In any event, any change of TAS of more than 5%/10 knots (whichever is greater) requires ATC notification, and if there's someone behind/ahead of you, don't expect to ATC to be happy if you boggle their spacing plan.

In addition, the EFC given in a holding clearance has nothing to do with when ATC actually expects to release you from the hold. An EFC has one purpose, and one purpose only -- to provide you a time to depart a hold if you lose comm while in the hold. Generally speaking, you will be released from the hold long before the EFC, and in any case, if your EFC arrives without further clearance, you are not repeat not permitted to depart the hold unless you've lost comm.

Air Force One seems to arrive within seconds of the ETA, at the chocks. Do you suppose they chalk it up to luck or superior flight planning skill?
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Neither -- it's all about traffic priority. ATC pushes everyone else out of the way to let AF1 do whatever it wants. They don't do that for you or me. The only folks ATC wants to be deciding whether folks slow down or speed up to meet gate times is ATC, because usually nobody else has the big picture.

That said, the FAA is running a test program with UPS to allow UPS jets in the middle of the night to work their own spacing into Louisville. The aircraft are all equipped with special data links so everyone can see where they fit into the flow and space themselves accordingly. Since nobody else is flying into SDF at those hours, it works, but it wouldn't work at ORD on a Monday morning unless everyone has the equipment, and they don't.
 
I would think flying IFR single pilot is taxing and I have never honestly done it so I might say I don't have time for this if I ever get into this situation. There is a way if you do have about three to five seconds to spare after your turned onto final. Read your ground speed and the IAP will usually tell you how far you have to go and bust out the E6B. :( Remember to read your speed at the 32 not the 60 mark which will give you more accurate times. Or before your turn on final, tune WX find the wind speed and work your magic on the back but I've found that time isn't as accurate.
 
Dean V said:
Please explain this further. I have yet to hear ATC chastize a pilot for not remaining within 5%/10kt of the airspeed filed in a flight plan
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Whether you've heard it or not, it's a required report. See AIM 5-3-3(e) and 5-5-9.

nor have I ever been issued a speed change for a spacing plan enroute.
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Listen to ATC talk to the jets in the high altitude structure -- it happens all the time.

While airspeed changes are included as a report in the AIM, it is not requied specifcally under required reports in Part 91.
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Well, by that argument, neither are any of the other "required" reports listed in the AIM 5-3-3. Yes, the AIM says only "should," but failing to give those reports only makes life more difficult for everyone concerned. If you want to get in a regulatory fight over that, be my guest, but I guarantee that you'll bust an IR practical test or IPC if you don't make them.
 
Areeda said:
Background: I'm doing oral prep for an insturment rating with another instructor's student for his retest.

I'm looking for a reference to how to determine the time from FAF to MAP.

I believe you estimate your ground speed and interpolate the timing table at the bottom of the approach plate. He says, you take the time from the next higher speed.

I've checked the books I have available (AIM, Instrument Flying Handbook, Instrument Procedures Handbook, Jeppesen Instrument/Commercial, PIC, Kirshner, Machado [can't find Gardner]) and I can't find anything on how to determine the time.

I tried to explain the error if you round 100 kts up to 120 can result in declaring missed early but all I get is "I was taught to round my speed up to the next value in the table".

A reference would really help. I have to admit, I have nothing authoritative to back my position either.

Thanks
Joe
Click to expand...

Just calculate the time difference, or do like I do and fly the approach at one of the listed speeds. That's why we practice controlled descents isn't it?
 
tonycondon said:
most of my students are busy enough flying the approach, and for most of them, mental math is something crazy people like me do. you should see some of the blank stares i get when we are doing ground on perf charts or w&b. "I get a calculator for the written, right?"
Click to expand...

Tell them "No, this is basic math, and you really NEED to learn how to do it quick and dirty in your head. This is where the rubber meets the road and where math means living or dying. Welcome to Aviation." If you can't do quick and dirty arithmetic in your head, you shouldn't probably be flying. I'm really tired of the "Anyone can be a pilot" attitude.
 
Funny, I have a student who has a hard time wrapping his brain around this, too. He finds it too hard to believe that interpolating a GS and time to MAP is acceptable. After all, his handheld GPS gives him an exact GS.

So here's his follow-up question: Can he use the GS from his handheld GPS in this instance? My answer was "not during the practical test." Plus, the basic skills need to be there in case it goes kaput.

I've seen this before with other pilots that use handheld GPS extensively. When you have a magic box that tells you everything you need to know, it's hard to shut it off and then make decisions from your best guess.
 
rcaligan said:
Funny, I have a student who has a hard time wrapping his brain around this, too. He finds it too hard to believe that interpolating a GS and time to MAP is acceptable. After all, his handheld GPS gives him an exact GS.

So here's his follow-up question: Can he use the GS from his handheld GPS in this instance? My answer was "not during the practical test." Plus, the basic skills need to be there in case it goes kaput.

I've seen this before with other pilots that use handheld GPS extensively. When you have a magic box that tells you everything you need to know, it's hard to shut it off and then make decisions from your best guess.
Click to expand...

JOOC, how does one come up with a GS on approach accurate to within +/- 15 Kt without a "magic box"?
 
rcaligan said:
So here's his follow-up question: Can he use the GS from his handheld GPS in this instance?
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Sure, until the examiner tells him the batteries died, which is what will happen if the examiner thinks he's too reliant on it. OTOH, if that's the only thing for which he uses it, the examiner will probably compliment him on his ingenuity and say no more about it.
 
lancefisher said:
JOOC, how does one come up with a GS on approach accurate to within +/- 15 Kt without a "magic box"?
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Plus or minus 15 knots is a pretty big window. But if there's an ASOS at the destination airport, I just do some quick and dirty math based on how much headwind/tailwind/crosswind I have. If I have a 10 knot headwind, I'll use the time for 90 knots and fly the final approach segment at 100 KIAS. If it's a 10 knot quartering headwind, I'll guess somewhere between 1-10 based on the angle.

It's not perfect, but close enough for me.
 
rcaligan said:
Plus or minus 15 knots is a pretty big window. But if there's an ASOS at the destination airport, I just do some quick and dirty math based on how much headwind/tailwind/crosswind I have. If I have a 10 knot headwind, I'll use the time for 90 knots and fly the final approach segment at 100 KIAS. If it's a 10 knot quartering headwind, I'll guess somewhere between 1-10 based on the angle.

It's not perfect, but close enough for me.
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I have seen *many* days with dramatic differences in wind from the surface to pattern altitude. It'd be pretty hard to detect this in actual and the ASOS could result in some very unrealistic numbers. To the point that it could be dangerous.

It's not everyday that this would be a problem but it is common enough that I wouldn't put much thought towards surface winds while calculating my ground speed at anything but the surface.
 
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Crikey, just ast ATC for your ground speed.

Lacking that, you should have a fair idea of your times between fixes along your route.

I don't fly hard IFR in a fast mover so maybe I don't understand how a pilot couldn't have time to follow along (keep up) during the flight.
 
Richard said:
Crikey, just ast ATC for your ground speed.
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That only works when you're in radar contact and talking to a controller with a scope. Generally you're turned over to tower at or before the FAF, and tower doesn't always have a radar scope. If it's a nontowered airport, radar service is generally terminated and you're switched to CTAF before the FAF. Also, at many airports (like my home 'drome), you're below of radar coverage once you leave PT altitude.

Lacking that, you should have a fair idea of your times between fixes along your route.
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That only helps if you're flying approach at the same airspeed and altitude as your enroute portion -- not a common situation. As noted above, winds usually change dramatically from cruise to the final approach segment.

I don't fly hard IFR in a fast mover so maybe I don't understand how a pilot couldn't have time to follow along (keep up) during the flight.
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The situation is unchanged for slow movers -- in fact, wind is a bigger factor in timing as you fly slower.
 
drizzt76 said:
I would think flying IFR single pilot is taxing and I have never honestly done it so I might say I don't have time for this if I ever get into this situation. There is a way if you do have about three to five seconds to spare after your turned onto final. Read your ground speed and the IAP will usually tell you how far you have to go and bust out the E6B. :( Remember to read your speed at the 32 not the 60 mark which will give you more accurate times. Or before your turn on final, tune WX find the wind speed and work your magic on the back but I've found that time isn't as accurate.
Click to expand...

Don't know if I would want to be breaking out the wizz wheel and spinning a ground speed while shooting an approach to minimums. Guesstimating a groundspeed (or using a GPS/Loran/DME groundspeed) and interpolating a time gets me close enough for government work.
 
Tim,

If you are handling the E6B after the FAF it's too late for that anyway....you have enough going on. A solid interpolarization would work. I will admit that flipping and twisting the wizz around will give you spatial d in your desk chair.
 
Ron Levy said:
Military bombers do that to make designated time on target (TOT), but that's an issue of coordinating attacks by multiple aircraft without biffing each other. However, I have never, ever heard of anyone in the civilian world adjusting speed to make flight plan ETA. That's a good way to throw a big monkey wrench in the controllers' works.
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You are confusing what I am saying. That seems to happen a lot here. People trying to find things wrong to jump on. What I am saying is that you should already be familiar with the adjustments from IAS to TAS and GS because you need to report if you are going to miss your next reporting point, and you need to know you are actually on your assigned speed. Not adjust speed for ETA, but adjust IAS to get your TAS within 10 kts or 5%. How do you know without knowing the IAS to TAS and GS adjustment? The point at which I suggested adjusting air speed was to fly a published approach speed on an approach plate for the published times, for simplicity. That is all I suggested. Once you are out of the enroute portion of the flight, you no longer need to fly your enroute cruise airspeed - or so I have understood. During the approach, make it as simple as you can. Why interpolate times when you can fly a published GS and the times are on the approach plate?
 
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Dwight B. Van Zanen said:
You are confusing what I am saying. That seems to happen a lot here. People trying to find things wrong to jump on. What I am saying is that you should already be familiar with the adjustments from IAS to TAS and GS because you need to report if you are going to miss your next reporting point, and you need to know you are actually on your assigned speed. Not adjust speed for ETA, but adjust IAS to get your TAS within 10 kts or 5%. How do you know without knowing the IAS to TAS and GS adjustment?
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Well, if you fly your planned speed, there's nothing to adjust, since you should have figured that IAS/TAS computation into your flight planning, and filed the TAS you plan to fly. As for making good your estimated time to the next point in a nonradar environment, most folks just report an updated ETA rather than change speed to make the original estimate good, although I suppose that if you're ahead of schedule, and the controller tells you that you'll have to hold if you arrive early, slowing down makes sense. But in a radar environment, the controller will let you know if you're running up someone's back, and give you a vector for spacing or speed adjustment as needed -- BTDT (e.g., "Tiger 22RL, reduce speed 10 knots for traffic you're following.").

The point at which I suggested adjusting air speed was to fly a published approach speed on an approach plate for the published times, for simplicity. That is all I suggested. Once you are out of the enroute portion of the flight, you no longer need to fly your enroute cruise airspeed - or so I have understood. During the approach, make it as simple as you can. Why interpolate times when you can fly a published GS and the times are on the approach plate?
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In order to have your plane at its optimal approach speed (Vref, for the big boys) on the approach. If I'm flying into a 25 knot headwind, I do not wish to fly the approach at 115 KTAS just to put myself in the 90 KGS time window -- I'd rather just recompute for 65 KGS and fly the plane as it likes being flown.
 
Ron Levy said:
Well, if you fly your planned speed, there's nothing to adjust, since you should have figured that IAS/TAS computation into your flight planning, and filed the TAS you plan to fly. As for making good your estimated time to the next point in a nonradar environment, most folks just report an updated ETA rather than change speed to make the original estimate good, although I suppose that if you're ahead of schedule, and the controller tells you that you'll have to hold if you arrive early, slowing down makes sense. But in a radar environment, the controller will let you know if you're running up someone's back, and give you a vector for spacing or speed adjustment as needed -- BTDT (e.g., "Tiger 22RL, reduce speed 10 knots for traffic you're following.").
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Will you just READ MY POST! I am not suggesting you "adjust" your TAS in the enroute phase of the flight. Just that you need to know your TAS and GS. My point has been and still is, you don't KNOW if you are going to make your estimated time to the next reporting point, and you don't KNOW you are on your planned TAS, unless you have updated your TAS and GS calculations along the way for any weather and wind changes that may have occured. So if you have done that, you already are aware of the IAS to CAS to TAS to GS adjustments that are needed in the weather you are flying in as you get to your destination and begin an approach. Why do you insist on interpreting this as some kind of ETA speed change thing? It is nothing of the kind.
 
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Ron Levy said:
In order to have your plane at its optimal approach speed (Vref, for the big boys) on the approach. If I'm flying into a 25 knot headwind, I do not wish to fly the approach at 115 KTAS just to put myself in the 90 KGS time window -- I'd rather just recompute for 65 KGS and fly the plane as it likes being flown.
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I guess you can fly your plane any way you want to, and make any computations you need to fly it that way. But I suspect that if your plane "likes" 90kts TAS on approach (65+25) it will fly just fine at 85kts. Which means with a 25kt headwind you could fly the 60KGS times with no computations required. Entirely your choice whether flying at 90kts TAS vs. 85kts TAS is worth the trouble of computing for 65KGS vs the 60KGS that is published on the approach plate.

My plane "likes being flown" at whatever speed I want to fly it, from about 55kts to a little over 120kts. It likes short final at about 70kts, but that is a different issue.

With a headwind under 20kts, I can fly the approach at 90KGS and when I pop out to visual on short final I can add flaps and reduce power as needed to get to 70kts well before the flare. With a headwind over 20 kts, I can fly the approach at 60KGS and when I pop out to visual on short final I can add flaps and reduce power as needed to get to 70 kts well before the flare.

I don't fly the "big boys". For what I fly, this works just fine. You do whatever you want. My point was, and still is, if you want to make things simple in IMC you can fly a published GS from the approach plate. Once you pop out to visual you are going to be making speed, flap and pitch changes anyway. I am not flying on instruments only, right down to the runway. That is what the MAP is all about.
 
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Word-for-word from the Instrument Procedures Handbook FAA-H-8261-1A page 5-33:

"On some nonprecision approaches, the MAP is given as a fixed distance with an associated time from the FAF to the MAP based on the groundspeed of the aircraft. A table on the lower right hand side of the approach chart shows the distance in NM from the FAF to the MAP and the time it takes at specific groundspeeds, given in 30-knot increments. Pilots must determine the approximate groundspeed and time based on the approach speed and true airspeed of the aircraft and the current winds along the final approach course. A clock or stopwatch should be started at the FAF of an approach requiring this method."
 
Maybe I'm just simple minded. I'm not very good at complicated stuff
while trying to fly an approach. Make a little chart and put it on your
clipboard with the time at your approach speed minus 5 knot
increments for 4, 4 1/2, 5, 5 1/2 miles. Just pick the one that works.

RT
 
RogerT said:
Maybe I'm just simple minded. I'm not very good at complicated stuff while trying to fly an approach. Make a little chart and put it on your clipboard with the time at your approach speed minus 5 knot
increments for 4, 4 1/2, 5, 5 1/2 miles. Just pick the one that works.

RT
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I like "simple minded" stuff when flying, Roger. Your idea of a chart is a good one. I whipped up one in Excel. Something like this??

http://spreadsheets.google.com/ccc?key=p536cZXPM8qVeKkCg55qGaw&hl=en_US

If I understand what you're saying, you fly your normal indicated approach speed, i.e., 90 knots or whatever... glance at the GPS to get the current groundspeed when you're stablized for the approach, and pick the closest matching time for the closest FAF to MAP distance (and you already know what distance column you're going to use, from your approach briefing).
 
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