When Not to Aim for the Numbers

[RotorAndWing]

Unless I'm not comprehending, it appears that you are are adding the requirement that a "stabilized approach" include power (how much? who knows) and that a slip cannot not be accepted as "stabilized."

Exactly.

I read it -- a while before you posted it, and took it for what it was worth -- an expectation that the approach would not include large excusrions from an ideal glidepath or airspeed once the turn to final is made.

You are correct.

 

[brcase]

<snip>
All these people who routinely do power off (idle) approaches from abeam the numbers must fly at airports without much traffic.

Geez I actually responded to this thread...

It is actually easy, you just delay pulling the power until you are at a position where you can glide to the runway. Following the C-150 on a 4 mile final you may be on final at 1000 feet before you reduce power to idle. The whole point of power off approachs it to learn how to adjust the pattern and airplane configuration so when your airplane becomes a glider it is not really an emergency but rather just another normal landing.

Brian

 

[gismo]

Unless the engine analyzers I've seen show that were all lying to me, I've seen enough actual data to know otherwise.
That gets you well over 100 deg/min.

My experience with analyzers is what I posted, but I may have gone too far by suggesting that no airplanes will see more than 50-60°F/min on a power off final. This might depend on how hot the cylinder heads were prior to beginning the descent from the pattern. I'm wondering if your experience involves a full power climbout followed by a power off approach? I suspect that when I've seen the minimal cooling rates I mentioned, the engines were already fairly cool from a long medium power descent prior to entering the pattern. I also believe that every case I can recall was in a plane with cowl flaps which might also make a difference assuming they were closed on the approach.

 

[Niladri]

Unless I'm not comprehending, it appears that you are are adding the requirement that a "stabilized approach" include power (how much? who knows) and that a slip cannot not be accepted as "stabilized."

So how do you fly a stabilized crosswind landing??

How about this... I am going out on a limb here...: A stabilized approach being one that maintains, within allowable limits (debate ... debate...) the path of the center of gravity of the aircraft along (a) the runway center line and (b) glide path determined by the runway landing aid (VASI, PAPI, ILS whatever...), and in the absence of a landing aid a glide slope which in the pilot's best judgment is steep enough to clear terrain obstructions in the flightpath transitioning to shallow enough to allow transition to level flight at recommended landing speed without subjecting the aircraft in excess of... CFI's please weigh in here... x Gs, with or without power during all or part of the approach and compensating for wind.

Brickbats, anyone?

- Niladri
Free Info for Student Pilots

 

[dmccormack]

How about this... I am going out on a limb here...: A stabilized approach being one that maintains, within allowable limits (debate ... debate...) the path of the center of gravity of the aircraft along (a) the runway center line and (b) glide path determined by the runway landing aid (VASI, PAPI, ILS whatever...), and in the absence of a landing aid a glide slope which in the pilot's best judgment is steep enough to clear terrain obstructions in the flightpath transitioning to shallow enough to allow transition to level flight at recommended landing speed without subjecting the aircraft in excess of... CFI's please weigh in here... x Gs, with or without power during all or part of the approach and compensating for wind.

Brickbats, anyone?

- Niladri
Free Info for Student Pilots

Short answer?

No.

 

[azure]

Pilots operating propeller driven aircraft should also maintain a stable speed and flightpath on final approach. A stabilized approach must be established before descending below the following minimum stabilized approach heights:

* 500 feet above the airport elevation during VFR or visual approaches and during straight-in instrument approaches in VFR weather conditions
* MDA or 500 feet above airport elevation, whichever is lower, if a circling maneuver is to be conducted after completing an instrument approach
* 1,000 feet above the airport or touchdown zone elevation during any straight-in instrument approach in instrument flight conditions
* 1,000 feet above the airport during contact approaches.

Hmmm... the third bullet makes me wonder whether the DPE could fail me for flying the ILS clean (i.e., no flaps) down to DA in my club 172, since I then have to transition to landing configuration at 200-odd feet above TDZE, which is a heckuva lot less than 1000 feet.

Then again, the advisory only says explicitly that being in "an approved landing configuration" is part of a stabilised approach for turbojet aircraft. And technically, no flaps is also "an approved landing configuration" in a 172, though it's not the landing configuration that I normally choose.

But it still makes me wonder...

 

[Everskyward]

Then again, the advisory only says explicitly that being in "an approved landing configuration" is part of a stabilised approach for turbojet aircraft. And technically, no flaps is also "an approved landing configuration" in a 172, though it's not the landing configuration that I normally choose.

But it still makes me wonder...

I wouldn't worry about what the definition for stabilized approach is in a turbojet if you are not flying one. Just for the sake of knowledge is fine, but you don't need to fly your 172 that way. IMHO.

 

[dmccormack]

I wouldn't worry about what the definition for stabilized approach is in a turbojet if you are not flying one. Just for the sake of knowledge is fine, but you don't need to fly your 172 that way. IMHO.

EXACTLY!!!!

THANK YOU for making the point!

A 172 is NOT a Transport Category Turbine aircraft.

Nevertheless, a "stabilized approach" is certainly preferable for an instrument approach in any fixed wing, since minimal inputs once established is much easier.

Once you transition to visual, the ideal would be to not change nuttin all the way to 20' AGL. But reality is you do what it takes to land.

 

[azure]

Right, it's the part about all this being highly recommended for propeller-driven aircraft as well that had me worried.

My approaches are stabilised all right (well, when I don't mess up anyway), but then I have to do some work to get ready to land. Luckily, most runways with an ILS are plenty long.

This is only in that particular 172 though, because of the airspeed limitation on flaps. In most other planes I'd be flying, I would add the first notch of flaps while being vectored.

 

[dmccormack]

Right, it's the part about all this being highly recommended for propeller-driven aircraft as well that had me worried.

My approaches are stabilised all right (well, when I don't mess up anyway), but then I have to do some work to get ready to land. Luckily, most runways with an ILS are plenty long.

This is only in that particular 172 though, because of the airspeed limitation on flaps. In most other planes I'd be flying, I would add the first notch of flaps while being vectored.

Hmmm...

I don't like slowly crawling around until established. Even in a slippery airplane (a Bonanza or Mooney), you really don't have to be too concerned with flaps and gear until established. Then power reduction to some pre-determined value, pull back a tiny bit to maintain level flight, slow down to target approach speed, trim to that speed.

Most experienced complex folks recommend gear then flaps at the FAF. You don't want to add flaps before gear is down as you can be lulled into thinking gear is down when it ain't.

Plus gear down with no other change usually gives you the ILS descent rate (give or take an inch of MAP).

In a fixed gear, low power plus flaps (10-20 degrees) usually provides enough drag to induce descent at the same speed you achieved after getting established.

So moral of the story is to go out and test various Power, Attitude Configuration (PAC) combinations until you know what will give you 450 FPM descent at 90 KIAS (and so on).

Only after you master this do you mess around with keep the speed up approaches, etc. Then you figure out the PAC for that scenario.

 

[Trapper John]

EXACTLY!!!!

THANK YOU for making the point!

A 172 is NOT a Transport Category Turbine aircraft.

Nevertheless, a "stabilized approach" is certainly preferable for an instrument approach in any fixed wing, since minimal inputs once established is much easier.

Once you transition to visual, the ideal would be to not change nuttin all the way to 20' AGL. But reality is you do what it takes to land.

But what if there's a dead horse on the runway?

Should we be asking, "Any turbojets in the pattern, please advise"?


Trapper John

 

[dmccormack]

But what if there's a dead horse on the runway?

Should we be asking, "Any turbojets in the pattern, please advise"?


Trapper John

Be my guest.

Just be sure to be stabilized when you ask.

 

[poadeleted20]

Can someone post the link to the definition of a stabilized approach again so we don't have to read back through 190+ posts to find it.

The information Dan quotes is not directed towards light aircraft in the traffic pattern. The stablized VFR approach is discussed in the FAA pamphlet "On Landings, Part I."

 

[poadeleted20]

I'm wondering if your experience involves a full power climbout followed by a power off approach?

No, it doesn't. As I said, I've seen the alarm on reducing as little as from 2250 to 1850 on downwind after arriving in the pattern. I've also seen it in the pattern when going from full power climbout to 1900 for downwind.

 

[PittsDriver]

The information Dan quotes is not directed towards light aircraft in the traffic pattern. The stablized VFR approach is discussed in the FAA pamphlet "On Landings, Part I."

Thank you Ron.

 

[azure]

Hmmm...

I don't like slowly crawling around until established. Even in a slippery airplane (a Bonanza or Mooney), you really don't have to be too concerned with flaps and gear until established. Then power reduction to some pre-determined value, pull back a tiny bit to maintain level flight, slow down to target approach speed, trim to that speed.

I flew a Cardinal RG for about 3 years as a non-equity partner. In that airplane (normal cruise 130 kts true), 21" MP and 10* flaps gave a very stable 100 kts in level flight. That's the way the "equity partner" (i.e. owner), who was instrument rated, flew all her approaches. Gear up until the FAF. The flaps were to slow down to a comfortable speed, and for stability.

Most experienced complex folks recommend gear then flaps at the FAF. You don't want to add flaps before gear is down as you can be lulled into thinking gear is down when it ain't.

I know these sound like famous last words, but it's hard for me to imagine gear upping a retract unless there was a lot of very unusual stuff going on. The gear in that Cardinal RG (and in a 182RG I've flown more recently) make such a difference in drag that there's no way I could imagine thinking it was down when it wasn't. And then, there's the gear warning horn.

Yes, I've seen the video of those two pilots wondering what the heck that sound was. Unbelievable.

Plus gear down with no other change usually gives you the ILS descent rate (give or take an inch of MAP).

Bingo. That's exactly how my partner flew an ILS: gear down at GS intercept, then no further changes in configuration until breaking out.

In a fixed gear, low power plus flaps (10-20 degrees) usually provides enough drag to induce descent at the same speed you achieved after getting established.

I don't like to add flaps at the FAF. If I reduce power from level flight without retrimming, airspeed stays roughly constant. But adding flaps both changes the airspeed in level flight and requires retrimming. So far at least, I've always wanted to be able to start a stable descent immediately at the FAF. If At untowered fields I often do add flaps for the descent, but I do it before reaching the FAF.

So moral of the story is to go out and test various Power, Attitude Configuration (PAC) combinations until you know what will give you 450 FPM descent at 90 KIAS (and so on).

Yup, I know I'll have to do that whenever I transition to a new airplane. In this 172, if I'm trimmed for 95 kts, 2200 RPM is level flight and 1900-1950 gives me a good ILS descent at 95 kts... without flaps. Roughly the same power settings work with 10 of flaps, but airspeed is more like 80 kts.

Only after you master this do you mess around with keep the speed up approaches, etc. Then you figure out the PAC for that scenario.

95 kts in a 172 seems to satisfy most controllers around here who want me to keep my speed up. I know I'm not ready to try to do it full out at 110 kts.

 

[dmccormack]

The information Dan quotes is not directed towards light aircraft in the traffic pattern. The stablized VFR approach is discussed in the FAA pamphlet "On Landings, Part I."

Interestingly enough, that document isn't found on the FAA site.

hmmmmm....

It's also interesting that the pamphlet does not require a "powered" approach -- only that:

At any targeted airspeed in any configuration, adding more power will make the glide path shallower; reducing power will make it steeper.

This inter-relationship means that any changes to one element in the "approach equation" must be compensated for by adjustments in the other.
So, after a glide path has been selected, the means of staying on it and maintaining your targeted airspeed can only be achieved by adjusting pitch and power together.

 

[gismo]

It's also interesting that the pamphlet does not require a "powered" approach -- only that:

Ya gotta admit that it would be difficult to reduce power if the throttle was already closed though, right?

 

[Clark1961]

Ya gotta admit that it would be difficult to reduce power if the throttle was already closed though, right?

Not at all, mag switch to off will further reduce power

 

[dmccormack]

Ya gotta admit that it would be difficult to reduce power if the throttle was already closed though, right?

Of course -- but altitude is energy.

In a light plane, is it better to have altitude or throttle in reserve?

Of course, this is not a hard and fast rule!

But power to idle on short final is not a stabilized approach...?

 

[gismo]

Not at all, mag switch to off will further reduce power

Sure, so would pulling the mixture, but I'll bet neither would be a good idea on short final while taking a PPL checkride.

 

[Clark1961]

Sure, so would pulling the mixture, but I'll bet neither would be a good idea on short final while taking a PPL checkride.

I'm'a thinkin' that pullin' mixture or switchin' off on the checkride would be bad things. Of course I'm not an expert on these things.

I wuz just addressin' the question of how to reduce power...

A few months ago I dealt with a problem of overspeed after finally getting a good tach installed. One comment from the A&P was that few folks thought about pulling back the blue lever as I did after noting the overspeed condition. That comment emphasized to me the need to think about the obvious solution of applying the direct control solution. As PIC we need to do what the situation requires. Nothing more or less.

 

[gismo]

I'm'a thinkin' that pullin' mixture or switchin' off on the checkride would be bad things. Of course I'm not an expert on these things.

I wuz just addressin' the question of how to reduce power...

A few months ago I dealt with a problem of overspeed after finally getting a good tach installed. One comment from the A&P was that few folks thought about pulling back the blue lever as I did after noting the overspeed condition. That comment emphasized to me the need to think about the obvious solution of applying the direct control solution. As PIC we need to do what the situation requires. Nothing more or less.

Well, in that case I'd definitely use the mixture control rather than the mag switch. For one thing with the mixture you can restore power as needed without blowing the exhaust off the engine and you can actually modulate the power somewhat by making the mixture almost too lean to run.

To reduce power sligthly on an idling engine you can switch to one mag, this is a technique I was taught to use in floatplanes where you want to minimize the thrust when you are approaching a dock.

 

[Clark1961]

Well, in that case I'd definitely use the mixture control rather than the mag switch. For one thing with the mixture you can restore power as needed without blowing the exhaust off the engine and you can actually modulate the power somewhat by making the mixture almost too lean to run.

To reduce power slightly on an idling engine you can switch to one mag, this is a technique I was taught to use in floatplanes where you want to minimize the thrust when you are approaching a dock.

See, it's easy to reduce power with the throttle closed...

 

[gismo]

See, it's easy to reduce power with the throttle closed...

Truly so, and I concede. I am curious to learn what increase in descent rate occurs in your airplane when switching one or both mags off on final with the throttle closed. What's your guess? I'm thinking that it would be 1-5 FPM per mag. Chances are this method won't lead to an overcontrol issue.

 

[Clark1961]

Since the Dakota descent is right quick already with 40 degrees of flaps I'd have to agree that one or both mags off would not result in overcontrol on descent rate increase. On the other hand, I've been told that in an engine out situation I can reduce the descent rate by setting the prop to coarse.

I suspect the actual number is related to the internal friction and oil pump loads for the engine so we could probably estimate the actual change in descent rate. If it takes two horsepower to drive the engine and accessories then the descent rate increase is on the order of 30 fpm. If the load required to drive the engine is five horsepower then the descent rate increase would be somewhere around 70 fpm. Propeller efficiency has been ignored in all computations.

Of course simply switching off one mag would result in very little increase in descent rate. Perhaps it can be estimated as one-tenth of a horsepower or a descent rate increase of 1.4 fpm (okay, I cheated, I just took your estimate and calculated the required change in horsepower. I really have no independent estimate of the horsepower reduction of switching a mag to off while at idle.)

One side benefit of this analysis approach is that it points toward a method to "throttle" the power reduction. If the alternator is left on then changing the electric load could help fine tune the glide path.

Is this the appropriate point to say something about not trying this at home?