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-   -   Selecting flaps during turns (https://www.pprune.org/tech-log/609439-selecting-flaps-during-turns.html)

Double Back 29th May 2018 18:08

Selecting flaps during turns
 
It was a loooong time ago during my ab initio training that we were instructed not to increase a flapsetting during turns, as the plane could stall because of this. (old Saab Safir with good a old biceps powered flap lever.) The argument was it could stall because of this, and because we all were Young lads we never questioned it.
Remarkably (it was around 1975) nearly all of the then group of instructors were ex-mil jocks.
Later I completely forgot about this, it never came back during training in any of my +/- 100 types I have flown.
Till last week when i needed a checkout on a C172 because of a medical stop of one Year and I was checked by an ex mil pilot who remarked my occasional flap setting change during turns. (with normal circuit speeds...)

I answered a bit laughing that was old and obsolete theory, but I did not convince him.

Anyone recognize this and give me some powder I can shoot back? :).
(btw I am at an extremely well relation with this chap, no worries!) :)

Richard

Greek God 29th May 2018 19:18

I have never heard of increasing flap during a turn decreasing the stall margin. Load factor possibly but stall??
Conversely, I have always been cautious of raising or decreasing flap in a turn and even now I dont like doing it without a healthy margin. These youngsters doing an SID with a close in turn requesting flap retraction 2 nanoseconds after Fr next.

pulse1 29th May 2018 19:39

I too seem to have picked up this concept somewhere in the past. My poor memory suggests that it was more due to the possibility of getting uneven flap deployment which would be more difficult to diagnose in a turn. Whatever the real reason, if there ever was one, it is still something I am conscious of while in the circuit.

suninmyeyes 29th May 2018 20:12

I was taught this during a UK PPL in the 1970's. Like the post above it was explained to me it was in case of uneven flap deployment causing a roll. When I instructed in the USA and enquired as to whether they had a similar policy I was told it was a quirk of the UK and that Piper and Cessna had no such opinion on the matter. Since then I have had no qualms on extending flaps in turns in GA aircraft.

B-757 29th May 2018 20:43


Originally Posted by Double Back (Post 10160034)
It was a loooong time ago during my ab initio training that we were instructed not to increase a flapsetting during turns, as the plane could stall because of this. (old Saab Safir with good a old biceps powered flap lever.) The argument was it could stall because of this, and because we all were Young lads we never questioned it.
Remarkably (it was around 1975) nearly all of the then group of instructors were ex-mil jocks.
Later I completely forgot about this, it never came back during training in any of my +/- 100 types I have flown.
Till last week when i needed a checkout on a C172 because of a medical stop of one Year and I was checked by an ex mil pilot who remarked my occasional flap setting change during turns. (with normal circuit speeds...)

I answered a bit laughing that was old and obsolete theory, but I did not convince him.

Anyone recognize this and give me some powder I can shoot back? :).
(btw I am at an extremely well relation with this chap, no worries!) :)

Richard

..All aircraft limitations can be found in the manuals..There is no such limitation in a C-172..
..This is a personal preference by your instructor..It may be a good technique (on some aircraft), but not a required procedure..

Fly safe,
B757

CaptainProp 29th May 2018 21:27

Itís not a limitation, it is taught, as already mentioned, more as a technique to avoid additional control problems in case of split flaps.

CP

Double Back 29th May 2018 21:37

Just to add, my experience includes 17 Yrs as commander on the B747-400 and I cannot remember ever having had this taught within our company. Yes there are procedures for asymmetrical flap movement but normally that happens at safe speeds within the retraction/extension speed range and they do not include a roll back to wings level item.
I only can recall one incident in a C172 with one of my fellow instructors in which the mechanism failed partially and it extended only one side. There is time and controllability enough to counteract the rolling moment with ailerons. He selected the single one back and made a flapless landing. A no brainer.

On the contrary, in Cessna aircraft I love to leave a (trimmed) downwind altitude with a 10 flap configuration by reducing power, rolling in for base and adding flaps in the turn. The power reduction and the increasing bank drop the nose (which is safe), and the flap increase somewhat recovers this downward nose movement, just a little nose Up trim needed an it stabilises on the base leg speed.
But that technique will not fit all a/c, likely a low winger will do different.

vapilot2004 29th May 2018 23:20

Stall speed: flap deployment generally increases the stall margin - turning, decreases it. Stall concerns would seem to be an unlikely training point, unless flap retraction was the concern. Meanwhile, flap asymmetry recognition is certainly reasonable in training.

The most obvious reason, perhaps: Deploying the flaps adds an extra layer to pitch trim for students. For most aircraft in standard approach configurations, the two actions cause changes in wing load and configuration where pitch requirements are opposite upon turn entry, and additive at rollout.

parabellum 30th May 2018 10:41

Not so much on light aircraft but from the BAC1-11 upwards I remember changing flap in a turn was not approved, (company), in case there was asymmetric movement of LE or trailing edge flaps. Don't recall stall speed being mentioned.

Mach E Avelli 30th May 2018 12:07

The mantra of not selecting flap in a turn most likely stemmed from WWII experience. Aircraft returning with damage sustained in battle did not need to pull any more surprises on their overwrought crews with asymmetric flap while turning.
However, almost every transport aircraft certified in the last 50 years has has asymmetry protection designed into the flap system. Light piston aircraft are not required to have such protection, though several light turboprops have it.
I have yet to read in official manufacturer documentation any prohibition on selecting flap in a turn on any civilian aircraft, be it Cessna or Boeing.
So, it seems this one needs to go to the OWT basket.

icemanalgeria 30th May 2018 16:39

I remember the same
 
I remember having the same thought when I started to fly Big jets (767). it didn't feel like good airmanship to select flap in the turn, I'd flown Golden Eagles etc before and my thoughts on those light twins were about getting flap asymmetry and late recognition.

Fursty Ferret 30th May 2018 17:39

Flaps decrease the margin to the stall compared to the clean wing at the same AoA, so itís plausible advice (if a bit paranoid). It's possible that they're thinking about an accelerated stall in a level turn, or of the lower wing stalling during a descending turn and leading to loss of control.

I think you'd need to be on the very limit of aircraft performance to even be close to those scenarios; a 30į turn only increases stalling speed by 7%.

In a transport category aircraft I wouldn't be concerned. There's a reason the slats sequence first during flap/slat extension.

Vessbot 30th May 2018 17:56

The right set of circumstances could come together as the flaps are deployed

1. in the middle of the turn, G is increased to tighten it up for unforeseen (but foreseeable) circumstances
2. extra drag from the G slows the plane
3. extra drag from the flaps slows the plane
4. pitching moment from the flaps slows the plane
5. crew is too distracted by #1 to act on the speed decay vis-a-vis power or trim, and too task saturated from everything. They only pull back more on the yoke to counteract the descending tendency, thus slowing the plane more

Planes have been stalled for stupider reasons than that, so I believe it.

ZeBedie 30th May 2018 21:01


Not so much on light aircraft but from the BAC1-11 upwards I remember changing flap in a turn was not approved, (company), in case there was asymmetric movement of LE or trailing edge flaps. Don't recall stall speed being mentioned.
The 1-11 didn't have any leading edge devices.

Mach E Avelli 31st May 2018 06:45

My recollection of the BAC 1-11 was no L.E. slats, but I think it did have flap asymmetry cut out. Big flaps had quite an effect on stall speed - all favourable - so I still can’t see why an operator would prohibit selecting them in a turn unless they expected to take a missile strike at some time.
Years ago I experienced flap asymmetry in a DC3. As full flap was selected there was a nasty cracking noise and the old beast rolled very rapidly. Although it was controllable, we guessed one side had fully retracted, while the other side had fully extended. We took a punt and selected flaps up. This stopped the rolling tendency. DC 3 flaps don’t affect the stall speed by much, but they sure make great spoilers!
The problem was a hydraulic actuater rod had sheared its end fitting.

JPAirbus 31st May 2018 07:42

Technically the flap deployment leads to an increased AoA (chord line "pitch up" while lowering trailing edge devices). At the same time flap deployment increases drag, which reduces TAS at a fixed power setting, which requires an even higher AoA to maintain the same lift.
Bank during a turn increases g-load demands and hence needs a higher AoA to produce the same lift at a given speed.
If you would have been riding at or near the max AoA prior initiating the turn you might get into trouble. If you are in a deceleration "flow" the adding of flaps might cancel out trim requirements during deceleration, hence leaving you with a "minimum effort manoeuver".
Split flaps are an additional problem. Thatīs harder to recognize and to handle in a turn (as compared to straight and level).

suninmyeyes 31st May 2018 15:21

There's also a 50% chance that if flap comes out asymmetrically in a turn it will roll you towards wings level! This thread started out about advice given to pilots of light aircraft but has moved on to cover modern airliners which have built in flap asymmetry protection and where no advice about avoiding deploying flaps in a turn has ever existed. Oh the joy in a 747 of working out whether it was a split flap or assymetric flap condition. I remember in simulator details Captain, Copilot and Flight Engineer writing on a piece of paper which one they thought it was. Frequently two would think it split and one assymetric.

RAT 5 31st May 2018 16:54

I hope no eager beaver CP's are reading this and think their SOP's need a revamp and tweaking update. There could be an endless list of things that people could dream up where they fall into the category of 'I wouldn't do that just incase something else goes wrong.'

I did come across a training captain from a neighbour airline whose personal opinion was to teach not setting the park brake when 'line up & wait' on the runway. Why? The brakes might stick on and you block the runway. Like I said the list could be endless if you let your imagination go wild.

Vessbot 31st May 2018 17:12

Sounds like the Good Idea Fairy visited!

vilas 31st May 2018 17:31

This practice was on archaic aircrafts like the DC3s may be some Russian aircraft like AN12(not sure). That too the concern was flap asymetry or runway on one side causing a rapid roll which may compromise aileron Control. But definitely not for stall considerstion. All today's aircraft have protections which make this concern superfluous and none of the airliner documents mention a word about it. Observing some obsolete caution because of habit is not the way to go.


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