Reducing flaps on short finals

[abgd]

0-40 are all valid landing settings on a 172, aren't they?

[Jetblu]

Many years ago flying an Arrow III : on finals, about 200 -300ft I pulled 3rd stage of flap. Suddenly, the spring mechanism went twang, the bar went to the floor and I lost all flap. All overcome with a touch of power and slight pitch up.

I've often played about with flap during different phases. A common short field take off technique was to pull 2 stages of flap at 50 knots and bounce her off.
Again, not in the POH but it works.

[Above The Clouds]

mixturerichlean
About 9 months ago I was a passenger in a C172 - we were a little high on approach so the pilot, on 20 degrees flap, selected full flaps (30 degrees) on short finals. At about 400 - 500 ft coming in to land the picture of the approach appeared that we would need some more power to take us to the threshold. Instead of adding power the pilot reduced flaps to 20 degrees which flattened the approach and we landed on the keys with no extra input of power.

It was the first time I had seen this technique and wondered if this was acceptable (ie safe) and wondered what might the pitfalls be if this practice is not safe.

I wonder if the OP or anyone else has considered that maybe this pilot was just practising a glide approach after simulating an engine failure downwind, that would make the above procedure a perfectly normal technique to obtain the correct touchdown point on your runway, or an off airport landing in a field, personally I practice glide approaches regularly for this reason.

[Pace]

The problem with messing around with flaps is selecting the wrong flap setting. Some are electric others are manually operated and the approach should be stabilised.
We use 1.3 times the stall in a given configuration so your stall speed will be different with zero flap to full flap and that needs to be considered.

Also to be considered is more aircraft specific, the type of flap and what settings are drag and what settings are more lift so it really comes down to knowing your aircraft.

If i had an engine failure took full flap and realised I had got it wrong and was going to come down short of my desired spot then sure get rid of that drag no matter whether its from flaps gear speed brakes etc.

I too used that technique of on a short take off leaving the aircraft clean and applying takeoff flap approaching VR. The real danger is missing the flap or taking the wrong degree of flap while your distracted bouncing along a field and really distraction is the key word as your full attention should be on the takeoff not looking down at flap settings

Pace

[Sillert,V.I.]

As others have said, you could one day find yourself in an emergency where this remains the last available option to extend the glide.

I'd say the technique of using flaps for energy management in a power-off approach is both safest & most useful in types with manual flaps where there is no significant pitch change with flap retraction; the tomahawk & hershey-bar cherokees are obvious examples. If your flaps are electrically-driven, in a real emergency, you'd likely want the master switch off before you reached the point where manipulating the flaps in this way would be helpful.

If you have the right type of aircraft, I think it's a technique worth practicing; the last thing you want to be doing in any emergency is trying something out for the first time -though, again as others have said, not with passengers.

It's also one way of achieving a precision spot landing from a glide approach. It's a long, long time ago now, but I remember using the technique (in a PA38) during this part of my initial PPL skills test - and since I passed, it must have been acceptable to my examiner.

But I'm struggling to think of a valid reason to retract the flaps during a normal powered approach - the throttle is your primary energy management
control.

[Crash one]

My aircraft has such manually operated flaps, it also has a separate trim tab operated by the flaps. Last Saturday was windy, and with a 25knot crosswind I elected to land at some 30deg to the runway heading with just 200metres to touch down and roll out between the edge of the grass and an avgas store. Closed the throttle at 200ft, flap lever in one hand stick in the other, I was able to touchdown within a foot or so of where I wanted it, stopped in 130metres.
Taxiing in it slowly described a very gentle ground loop through 180 deg, keeping the tail down I managed to keep it going all the way round. First for me, completely un commanded out of control but very gentle. I love that aircraft, it lets me get away with murder.
Electric flaps are a different thing altogether.

[fireflybob]

Perhaps we should also mention that it has been known for pilots to select gear rather than flaps in more complex aircraft.....

[Pull what]

Instead of adding power the pilot reduced flaps to 20 degrees which flattened the approach and we landed on the keys with no extra input of power.

Landed on the keys! Rather says it all doesn't it, does this pilot wear spurs as well by any chance?

Flaps on C172 are Fowler,

Slotted Fowler Flaps actually, or to be absolutely correct single slotted Fowler flaps

What your describing is a non standard event and not best practice, as is landing on the keys but for some pilots non standard events are part of their normal flying and you can read much more about them in AAIB bulletins or here on Pprune.

The most important thing on the latter stage of the approach is to fully stabilise the approach in that the speed should be on target, the aircraft should be on the correct glide path, on the centreline and correctly configured with landing flap(and trimmed). The stabilisation height should be a target but most GA pilots have never even heard of stabilisation height so its hardly surprising they dont make an attempt to stabilise the aircraft by the target height. We/I use 200 feet on our training aircraft and that also coincides with the obstructed runway solo go around decision height.

Lifting flap at 400-500 feet is not much different to selecting flaps full in terms of stabilisation, both are de stabilising but thats not really the point, the point is whatever you have done to destabilise the aircraft on the approach should have it affects negated by the stabilisation height.

As has already been mentioned should an abnormal or emergency event occur on final the decision to lift flap may be the best decision on the day and shouldn't be overshadowed by folklore. This however wasn't an abnormal or emergency event and neither was it best practice in my opinion but Ive seen some very fine pilots roll an aircraft at 400 feet on finals but as I said earlier he eventually found his way into an AAIB bulletin and the local crematorium(from stall/spin at altitude in a twin I might add, not final!)

You also asked by the way, " is it safe"? I've yet to find anyone who can define what safe is. I teach instructor candidates to be very careful and sparing in their use of the word safe as ‘safe’ cannot be measured. Many instructors say for instance, “I want to see the student fly a safe approach”. What they really mean is they want to feel safe but feeling safe isnt a measurable value. The parameters of the stabilisation height however are and they are measurable both by the student and the instructor.

[fujii]

Slotted Fowler Flaps actually, or to be absolutely correct single slotted Fowler flaps.

To be absolutely correct, "What your describing..." Should be "What you're describing"

[Sillert,V.I.]

Originally Posted by Pull what

The stabilisation height should be a target but most GA pilots have never even heard of stabilisation height so its hardly surprising they dont make an attempt to stabilise the aircraft by the target height. We/I use 200 feet on our training aircraft and that also coincides with the obstructed runway solo go around decision height.

My own view is that stabilisation height isn't relevant to operating a fixed gear SEP and there are many situations in which you simply can't be stable by 200ft. Landing on a short grass strip with obstructions on the centreline on short final is just one obvious example. The weather will frequently make a mockery of any attempt at stability (my local airport as I type this: EGHH 051350Z 22023G35KT 9999 BKN025 15/08 Q1000); on days like today I'd likely have the flaps up immediately after touchdown to keep a clear differentiation between air and ground ops.

I've frequently made a runway change below 200ft; it certainly used to be common practice at Wycombe for operational reasons (getting out of the way of Lord King's personal transport was one such occasion) and (in perfect weather) I've even made a very low level change from 24 to 27 at EHAM . (It's been awhile, but as I recall it went something like: "G-xx short final 24" "G-xx cleared land RWY 24" "Cleared land 24 be advised going international on departure G-xx" "Can you make 27" "Affirm" "G-xx cleared land RWY27 no need to acknowledge" ).

It's important to distinguish being stable with being in control. Light aircraft are generally very manoeuvrable and imposing stable approach criteria is just denying the pilot the advantages of this manoeuvrability. OTOH, if you are in any doubt at all about being in control on short final, then I'd agree a GA is called for, and I can see the merit of imposing a commit height for this.

I'd agree teaching a stabilised approach with a commit height makes sense if the student's short term aspirations are the RHS of a B738; but IMO it would be better if those folks were never let anywhere near an SEP and we made the most of their brief time in training by concentrating on matters relevant to airliner operation. Unlike some here, I've no issue about low time pilots in the RHS of an airliner if properly trained, but that training needs to be relevant to the job they're actually being asked to do.

[Above The Clouds]

Pull What
The most important thing on the latter stage of the approach is to fully stabilise the approach in that the speed should be on target, the aircraft should be on the correct glide path, on the centreline and correctly configured with landing flap

Oh my, another school teaching Airbus, Boeing approaches in a Cessna 150, I suppose everything will be fine so long as they are on the glide path at 10 miles fully configured, what ever happened to doing a visual approach visually.

[abgd]

On my home runway there's a big patch of sink that means that anybody on a stabilised approach at 200 feet is setting themselves up to land short.

I aim to fly every approach high and use sideslip to correct it. Partly because it's fun, and partly because it seems like a safer thing to do than to power in from miles out, with the risk of landing short if you have an engine failure.

Perhaps this is the real reason reducing flaps is a bad idea: there are better ways of controlling your glide angle. It's all very well using a mechanism that enables you to steepen your approach whenever you feel high, but at least if you aim to go in with a degree of slip you can reduce your glide angle without having to cope with all the trim changes and changes in stall speed that result from playing with the flaps.

[Sillert,V.I.]

I'd agree that slipping is perhaps more useful for those with some experience, though it also depends somewhat on aircraft type.

For example, I learned to fly on the PA-38, which has no signficant pitch change with flap movement; the stall speed difference between clean & dirty is 2kt & it has manually controlled flaps. My instructor was perfectly happy to let me practice glide approaches solo using the flaps as makeshift airbrakes. I was lucky enough to nail one just right for my final handling test.

The PA-38 slips well, too - but it bites hard if stalled with crossed controls & the instructors didn't encourage PPL students to slip it solo.

You'll never forget your first spin in a tommie.

[skyhighfallguy]

sillert vi

yes, tomahawk spins are a wakeup call. good idea to secure everything in back before you spin.

So many people write about engine failure, and certainly it can happen. So can spar failure.

but it is best to control a plane with primary flight controls. changing flaps seems like such a waste.

I've taught many people to fly and would not spend more than a few minutes of briefing on non standard ways to fly the plane. things like in an emergency like a stuck throttle, try using the mixture to control power, or if the ailerons fail, a little rudder and not too sharp a turn.

But flight path control with flaps belongs with things like ground signals to search aircraft. Worth talking about, but I would not make a student demonstrate the signal for injured person.

Wondering if the same flap jugglers even know the agreed upon signal from surface to air for injured person?

[Sillert,V.I.]

Originally Posted by skyhighfallguy

Wondering if the same flap jugglers even know the agreed upon signal from surface to air for injured person?

Isn't this the one where you lie flat on your back with your hands straight up above your head?

The one that really threw me was the first time I was asked to fly a circuit with the ASI covered up.

Sometimes I wonder if the training's more dangerous than the emergency.

But I stand firm on using flaps as airbrakes after engine failure. When the donk quits, you're flying a glider - and glider pilots routinely land using airbrakes for flight path control.

[9 lives]

But I stand firm on using flaps as airbrakes after engine failure.

I prefer to keep drag to a minimum following an engine failure. I will extend flaps for a landing as appropriate, and to increase drag appropriately on final approach. In the case of a forced approach, this would be very late in final, and not to be fiddled with after that. I'll be too focused on making a safe landing.

When the donk quits, you're flying a glider - and glider pilots routinely land using airbrakes for flight path control.

No, if an engine fails, you're flying a power plane without power, nearly none are equipped with airbrakes. A glider is a different type of aircraft, equipped differently, and to be flown differently.

Airbrakes are airbrakes, and flaps are flaps. Airbrakes are not designed to produce lift when they are applied. Airbrakes are intended to be modulated during a normal approach to create drag as desired without a lift change, flaps are very certainly not - they create both lift and drag in varying amounts per extension.

It is very poor practice to attempt to modulate lift with flaps. I agree that with a manual flap plane it is possible, but on final approach, silly, and, if not rather unsafe, at least needlessly distracting. For electric flaps, hopeless - they just don't move quickly enough.

If you feel that you need to reduce the flap extension on final, you should be abandoning that approach - you got it wrong.

The most important thing on the latter stage of the approach is to fully stabilise the approach in that the speed should be on target, the aircraft should be on the correct glide path, on the centreline and correctly configured with landing flap(and trimmed). The stabilisation height should be a target but most GA pilots have never even heard of stabilisation height so its hardly surprising they dont make an attempt to stabilise the aircraft by the target height. We/I use 200 feet on our training aircraft and that also coincides with the obstructed runway solo go around decision height.

This is much more simple than that. Stabilized, in the context of a GA aircraft approach has no numbers which make it right or wrong. It is simply, at every point in the approach, are you continuing what you are doing to make a smooth transition from flight to rolling down the runway. If you are continuing to descend, maintain a speed with reductions as needed, and configure the aircraft for landing, you have a stable approach. It could be steep or shallow, turning or straight - but it is not doing then undoing - it's continuing to do. You planned a "good" approach to demonstrate good airmanship, you're going to continue to do that, not start to undo it partway along! If you need to undo it, undo it properly and go around.

If you are flying a manual flap Cessna, with 40 extended, and you think to retract to 20, and miss, and get to zero, you'd better get full power in right away, 'cause you're dropping. That is anything but a stabilized approach.

There is no power plane flight manual which is going to include the modulation of flaps for "control" of the aircraft. Why would pilots think that this is a good idea - are they test pilots?

Lifting flap at 400-500 feet is not much different to selecting flaps full in terms of stabilisation, both are de stabilising but thats not really the point, the point is whatever you have done to destabilise the aircraft on the approach should have it affects negated by the stabilisation height.

I disagree. leaving the flap as is, or extending more as required, will continue the stable trend toward slowing and increasing lift as will be desirable as you enter the flare. Raising flaps reduces lift and drag, and not necessarily in proportion. As lift and drag, compared to "normal" flight, are desirable at the flare, reducing them during approach is destabilizing.

[skyhighfallguy]

good for you sillert



and if you use flaps to increase drag if your engine has really quit, well, do what you have to do to stay safe.

IT IS THE IDEA of using flaps instead of changing power settings that is not a good one.

[Pace]

Obviously it's bad practise to use flaps to popping them up and down like a drag control! Maybe it's having such a long history flying light twins where drag can be an enemy that I will only add drag when needed!
But here we are looking at an engine failure in a light single! Again drag should be only added when you need it and full flap when assured of a landing!
But pilots get it wrong sadly and lateral thinking in that situation should mean a pilot considers that extra drag and gets rid of it better that than to continue a speed glide profile into trees short of your landing spot!
I would not recommend the use of flaps in that manner in normal circumstances but would encourage pilots to be aware of that drag in emergencies

Pace

[ANOpax]

Grammar

Slotted Fowler Flaps actually, or to be absolutely correct single slotted Fowler flaps.

To be absolutely correct, "What your describing..." Should be "What you're describing"

I was once told that grammar is the difference between knowing your **** and knowing you're ****...

[oggers]

Obviously when reducing flap the aircraft will sink before the glide path flattens out so there is the potential to exacerbate an undershoot. But let's have a sense of proportion here, reducing flap from 30 to 20 above 400' in a C172 is safe, and it worked out nicely.