Question For The Experts - Use Of Flaps.
 

Flying schools typically require a training aircraft, say a Cessna 172, to fly a base leg around 65 knots, final around 60 knots to the threshold.

1.3 Vs is around 61 kts as Vs, power off, flaps down, is about 47 kts. Stall clean is around 55 kts from memory.

My question is as follows. Suppose you are on final, configured with full flaps for a landing, say just at 500ft. The engine coughs splutters and dies.

You are doing 65 knots. You judge you aren't going to make the field. Yes, I know I should be able to make the field, but that's moot just now. In addition at places like YMMB, some pretty flat approaches get taught in my opinion.

What is to stop you retracting flap to extend your glide?

I've never seen this discussed, let alone demonstrated or taught. The rule seems to be that once flaps are down they aren't going to be touched until you are on the ground. What am I missing here apart from the obvious issue of retracting flaps below the clean stall speed?

I've tried this at altitude in an approach configuration and their are no abrupt trim changes, etc.

The same issue relates to off field forced landings. Too much flap? Why not just retract some until your glide is what you want?

Handing over....

My school teaches that if you are going to fall short, then get the flap back up. I learnt this first time I did a glide approach in the 172. Took flap as per normal in the 152, but fell short. My instructor explained that it "flies like a brick with full flap compared to the 152, so if you need to take some flap up to extend the glide on final, then do so."

This was also said by the ATO when I did the PPL test recently. High on base and final, side slip in. Afterwards we went over the forced landing leg lengths and heights. During this he also said "if you find yourself falling short, then clean the flap up to help extend the glide"

His reason was, you only have one shot in a real emergency to get it in the field, so do what ever it takes!

So some places are teaching it to students, like me :}

I understand that the main reason for not retracting flap as a 'rule' would most likely be because this type of training is being provided to students who cannot be relied upon to manage the decision; so after consideration of the way humans work under pressure there is a 'more reasonable' chance of success with the flaps as they are etc because there is 'less' to do and it will likely lead to a lower controlled touch down speed.

By retracting the flap (large changes in configuration) there is a considerable sink below the current glide path followed by an increase in speed... supposing you survive the extra sink and don't hit anything at your increased speed, you will be able to glide further from where you are when best glide AoA is re-established. :}

In doing this at low altitude e.g. <500 AGL you risk misjudging the extra sink as this procedure would only be done with an optimistic view of a worsening situation; and you also risk the higher possibility of entering a spin or a stall and dying for sure.

At higher altitudes or with small changes in flap (e.g. 40 degrees up to 30 degrees on a C172) and with more experience your chances of success with this procedure are much better...:ok:

Rule 1. NEVER extend full flap until you are assured of a landing ! Pretty basic training......that doesn't seem to be taught anymore. This was also said by the ATO when I did the PPL test recently. High on base and final, side slip in. .....Pretty sure side slips in a 172 are a no no.

in the broad strokes, flap creates a bit more lift and a lot of drag, there is a reason gliders are very slippery! to minimise drag.
the flap is just another flight control at your disposal, use it as necessary for safe flight, if your falling short in a glide, lose the drag and glide thurther.. of course, at best glide speed, for L/D ratio.

Sorry, I should of clarified that I side sliped it to about 300 from about 600ft. No one has told me its a no no in the 172, though I try not to make a habbit of needing to!

I have only had to bring the flap back up once other than the time I was shown, and then it was only back from 30 to 20deg, so the attitude change was pretty small, and I felt more comfortable with my slope than before. I can full see how it would cause large sink going from full flap to none, I dont think I could ever see anyone doing that unless at great height.

I dont know if it makes any difference, but in the focred landing check list, we list flaps "as required" rather then "flaps full" which I heard once. I guess this lends itself more to being able to use them as need be before switching the master off?

First of all, you should not have 'full flap' in a C172 at 500 feet. I know they teach students these days to configure once they are established on final, however this will involve you 'dragging' it in all the way down final. This is 'poor' airmanship when considering the following aircraft, and a recipe for disaster if the situation you questioned happened for real. It may be okay at a GAAP (class D) aerodrome where everyone is doing the same thing, if you fly in the real world and do this with an RPT aeroplane up your behind, you could become an accident waiting to happen - especially if you were not making the required radio calls (which has happened to me on occasion).

Energy management is a crucial aspect of airmanship which is not taught. The trick is to fly the aeroplane all the way to the ground taking into account the current conditions without making large power changes, adding flap as required.

If you are following a slow aeroplane on downwind, don't fly wide, stay in gliding distance of the field, slow down and extend 10 degrees of flap. If you have the circuit to yourself, stay clean (not 3nm which seems normal for lighties these days) and extend flap as you require.

As a rule of thumb when I was flying piston VFR aeroplanes, I would not configure until 300 AGL (gliding distance of the threshold). Stabilized approach criteria is something you should be worrying about when you are flying an aeroplane where this is important.

In regards to your original question, you do as a Pilot what is required to fly your aeroplane safely in regards to the current situation. If this calls for raising flap to extend your glide to make the field - then do it. If this requires side slipping a C172 (which is prohibited in the POH with full flaps) then do it!

i think the poor airmanship side of it comes from a lack of situational awareness, i am capable of flying an approach at 30 kts with full flap, but if there is other traffic, then no way will i do it.

and whats the reason for NOT having full flap on a C152 at 500 ft?
full flap, plus a very steep approach into a tight field? then you can do a glide approach with full flap in any aircraft.

my greatest satisfaction is a when i can glide from base, yet still get full flap out and make a 30 kts landing right on my mark with 50 mtr landing roll and not touch the throttle.

I'm by no means an expert but PA39 summed up my point of view beautifully.

I was taught to have full flap out by about 400 feet, but it wasn't until my first Chief pilot pointed it out that i realized that this practice is "unsafe" in an engine failure situation. He suggested putting out full flap going over the fence, however I feel that's getting a bit late. I tend to throw out full flap when I'm assured I'll make it to the clearway, it leaves me enough time to stabilize the aircraft before the flare and if the donk did go U/S I'd be happy to put it in the undershoot if needed.

Because you have a couple of barn doors hanging out the back creating heaps of drag. This in turn requires you to use lots of power to maintain the profile. I've done my fair share of short field ALA operations and a standard approach profile IMO is the best way to get the job done.

Sometimes in the 172s/152s I'd leave 20 degrees out and land with that! Lets face it, they will land flapless in less than 400 meters!

Again! Only apply full flap when you know you are going to make it to the runway!

You wouldn't want to apply it before that, there's no need and no recovery if your thrust is lost.

Sunfish, you should give it a go. Practice up a bit higher and see what happens to your glide when you drop the flaps and re-apply them.

Question why it is unsafe. I assume most here are talking about a powered approach and any aircraft which loses power with or without flap is not going to make the powered aiming point, it will fall short.

Unless you are all conducting a glide approach from the start from some point late base and final you will not make the runway even clean unless you are aiming originally to land a good way in or at excessively high speed.

The benefit of having flap already out is that no configuration change is necessary and the touchdown speed off field will be much less.

Some larger and high performance singles have very little hope of making the runway even from a close downwind position, especially if there is a headwind to the field. The best method to teach is always know how far you can safely glide in any configuration and have a clear area to put down off airport in the event of total power loss. If you can safely make the runway do it, if not take the off field option. Focusing too much on stretching a glide back to the field results in stall spin accidents and its better to land in the rough upright (better still a reserve, padock or oval) at low speed than arrive at the boundary fence inverted and lacking control.

I think the CP should not even take off as at some point after lift off the aircraft would not be able to safely glide back to the runway after an engine failure.

Any sideslipping should be done with caution in most trainers, especially with lower fuel loads as it can lead to fuel interuptions.

In any case discussion on the point should also involve some practice. In a real emergency best to stick to what you know and have done rather than try something new.

A well known ex Kununurra CP used to demonstrate a turn back after take-off and expect his boys to be competent at the maneuver.

Whilst not always the best thing to do, it is another trick in the bag if it is ever required.

What absolute bullocks :ugh:

High performance singles are usually very slippery and glide far more efficiently than your average trainer.

The only exception to this is perhaps the mighty C207 and a Cherokee 6. (and the C209). However if you can't make the field from your downwind position, you should rethink your circuits IMO. Something like a PC12, a turnback after takeoff is a QRH maneuver.

Especially with the size of circuits that are promoted at the training fields these days. The circuits I flew, I would have been able to make the field in 90% of the circuit, and I kept this margin in every piston type I flew.

You can tell who the Grob/C152/C172 instructor is :zzz:

This one has been touched on before

Retracting Flaps

There's a bit of drivvle there, but post #7 gets to the point - I think this bloke knew what he was talking about.

Energy Management
 

Thank goodness for your wise words Green Goblin, I was really concerned with several of the posts.

Good to ask though - can't learn otherwise.

Energy management is exactly what it is in the smaller cessna's (et al)- each circuit can be different. Circuits should always be flown with possibility of engine failure in mind - too late when it has happened. Good practice for night circuits also.
Long time since I have flown the cessnas but I recall sideslipping with full flap prohibited in at least some single cessnas because
1. loss of elevator authority (due shielding)
2. gravitiy feed fuel system

:)

BA38 anyone?

pretty sure that had a major impact (retracting the flaps) in the outcome of that flight at LHR..

Another thing Sunny, you're flying a C172!

KISS!!!

1.3 VS, bla bla bla, FFS IT'S A C172!!

What I suggest you do, put a post it note over that ASI and fly looking out of the big thing called a window out the front. Use your primary and secondary flight controls to take you where you need to go.

If you really want to learn something, go do a tail wheel endo in a Moth. It will really sharpen up your flying skills and get you looking out of the front of the thing rather than trying to treat it like a heavy.

To put it another way :p KISS!

GG

No one is saying to disturb or conduct an unstabilised approach at all. They/we/I are just saying to not lower all your flap, until you are going to make it in.

The last 2 hours you have been above LSALT in cruise with time to plan your glide and off field landing. A failure on final approach in landing configuration will have you in the weeds before you probably comprehended the gravity of what was happening. This is the time you need to be prepared for an emergency.

A little bit of prior planning and personal SOPs help prepare your recovery in a future upset.

WHY oh why are you even discussing a "stabilised" approach??? It's an inappropriate notion when discussing the average GA airframe -up to & including most twins!!! You have neither the mass nor the inertia inherent in that mass to properly conduct a "stabilised" approach!!! Sure, there are elements of the stabilised approach paradigm that are utterly appropriate to GA airframes and pilot action, but the practice of attempting to "stabilise" the average GA approach is a complete nonsense!!!

Energy management is what GA drivers need to be concentrating on, along with appropriate circuit tracks and size. How many "pilots" do you see out there flying 747 circuits, dragging their airframe to the threshold with almost full power whilst sitting right on the back of the L/D curve in what is their interpretation of a "stabilised" approach?

Further, why not have full-flap in a C172 at 500'??? I used to have exactly that configuration most approaches in all sorts of conditions in some locations -in older 172's that had the 40 degree flaps too, not the more recently built 30 degree models. When you needed to get down, that was the way to do it. Admittedly, it's not always necessary, but when you need to use it, you need to know how -and don't try to overshoot from that configuration either. You'll wind-up as a smoking hole in the ground. Something like that will teach you about energy management in no time.