PPL PoF Question

[Binners93]

Morning Pruners!

So some advice would be much appreciated from those who have a sound understanding of all things PoF related, as one question that came up in my flight school had me scratching my head!

The question went along the lines of;

"You are flying at Va and pull back on the controls in an full abrupt manner, what is the first thing that will happen?"

a) You will enter a zoom climb.
b) You will cause structural damage to the control surfaces.
c) A silly irrelevant answer.
d) Another silly answer to add a forth option.

My first thought was that the question has specified Va, not Vfe. At Vfe, its my understanding that ANY control surface deflections are likely to cause structural damage, let alone a full pull back. I also was thinking about the lift principle and how an increase in the angle of attack and such a fast velocity is likely to climb the aircraft quite dramatically. I selected a), turns out it was b) .

Can any of you lend some advice with this one? I understand why structural damage will occur, but the questions asks what is the FIRST thing that will happen. I would have thought that if it was at Vfe, first thing to happen is a freefall skydive with the plane in tatters but at Va, a climb and then possible damage?

I know I'm probably missing something obvious so would greatly appreciate any assistance!

[Jan Olieslagers]

What's PoF?

[Binners93]

Originally Posted by Jan Olieslagers

"What's PoF?"

Principles of Flight

[Level Attitude]

Although a) is possible that is not the first thing that will happen - need a change in AoA first (which would not be instantaneous) and before the abrupt control deflection causes an AoA change it would (possibly) cause damage.

b) is the correct answer by definition:

In aviation, the maneuvering speed of an aircraft is an airspeed limitation selected by the designer of the aircraft. At speeds close to, and faster than, the maneuvering speed, full deflection of any flight control surface should not be attempted because of the risk of damage to the aircraft structure

VA = Design maneuvering speed (also called corner[ing] speed[citation needed]). This is the speed above which it is unwise to make full application of any single flight control (or "pull to the stops") as it may generate a force greater than the aircraft's structural limitations

NB:

VFE = Maximum flap extended speed

Nothing to do with control deflections.
(were you confusing it with VNE)

Above definitions courtesy of Wikipedia, that well known authoritative aviation reference.

[dublinpilot]

I'd no idea what PoF is either.

But you shouldn't consider VA as a strict line, and one side you're perfectly 1kt below and will instantly fall apart 1kt above.

Approach it with caution. You'll probably get away a bit over, but don't be sure. Slightly over damage is likely to be minor rather than total destruction.

So I think that the answer is enter a zoom climb and risk damage.

[BackPacker]

At Va, full and abrupt control deflection should not lead to any damage, as the aircraft will stall before the forces increase beyond the design limit. So AFAIC the correct answer is "the aircraft will stall".

If you want it to enter a zoom climb, you should not use full, but only partial deflection. It's done by feel and by listening to the stall warner: You haul back on the controls just so that the stall warner sounds, but no so much that the aircraft actually stalls.

(And note that Va decreases as the mass of the aircraft decreases. So if you're flying solo, your Va will be lower than when flying 2-up for instance.)

At Vfe, its my understanding that ANY control surface deflections are likely to cause structural damage,

Nope. Vfe is just your flaps extend speed. You can't extend flaps above this speed, since they might be blown off. (Some aircraft, including the C172, have multiple flap extension speeds, where partial flaps can be extended at a higher speed than full flaps. Vfe will then be the speed where full flaps are possible.) This has nothing to do with control surface deflection - although the majority of aircraft do limit the G-forces that are sustainable with flaps extended. For instance, an aircraft might be capable of +4 and -1 G with flaps retracted, but only +2 and 0G with flaps extended.

If you're confusing Vfe with Vne, then again you're not there yet. Vne, the "never exceed" speed is typically not determined by control deflection/g forces/stall speed like Va, but is determined by flutter tests and similar. So at Vne it is still possible to use some careful measure of control deflection - otherwise you'd never be able to pull out of the Vne dive. Sure, you can't use full deflection - you're above Va after all - but some measure of control deflection is certainly possible.

[flying apprentice]

When did Zoom Climb enter PoF ? I did ATPLs 8 years ago and never heard of it.

[Binners93]

Apologies all, I meant Vne - not Vfe

Thank for the clarification, that makes more sense now!

[Above The Clouds]

If the designers and test pilots have done everything correctly then the aircraft should stall first to avoid structural damage.

[phiggsbroadband]

quote...
If the designers and test pilots have done everything correctly then the aircraft should stall first to avoid structural damage.


That design works Ok up to Va, where the 3G loads cannot be achieved, limited by stalling.


However beyond Va, I am afraid it doesn't work like that... Some numpty is sure to do a 'Watch This!' vertical dive from 12,000ft and then wonder why the wings folded when he pulled up.

[Above The Clouds]

"You are flying at Va and pull back on the controls in an full abrupt manner, what is the first thing that will happen?"

The OP states flying "at" VA, therefore in theory the aircraft should still stall within the 3g limit because he is not exceeding VA, it is hypothetical, but only answering the OP question in the literal sense, if some numpty is daft enough to put it to a test from 12000' then good luck to them.

There is a % safety buffer built in, for the same reason why flying 1 kt over VNE the aircraft does not immediately disintegrate.

[Level Attitude]

If the designers and test pilots have done everything correctly then the aircraft should stall first to avoid structural damage.

At Va, full and abrupt control deflection should not lead to any damage, as the aircraft will stall before the forces increase beyond the design limit. So AFAIC the correct answer is "the aircraft will stall".

But is Va the speed:
i) "below which the aircraft will stall, if full and abrupt control inputs are applied, before structural damage will occur." ?
or
ii) "above which structural damage will occur, if full and abrupt control inputs are applied, before the aircraft will stall." ?

Binners93 original Post seems (to me) to be asking how to answer a written theoretical question so "should" or "might" are not very helpful in this context.

In the question posed by Binners93 the aircraft is flying (exactly) at Va.
I believe the definition of Va is i) above, therefore b) would be the correct answer to the question asked.
(especially as "the aircraft will stall" is not given as a possible answer)

[Shaggy Sheep Driver]

And remember that the point that stall AoA is reached, and therefore the value of VA, varies with weight.

[BackPacker]

therefore b) would be the correct answer to the question asked.

I don't agree. At Va, you haul back at the controls. That means the aircraft stalls exactly at the moment the maximum design forces are reached. For which the aircraft is designed (duh). So there is no structural damage, so B is not the correct answer.

B is not the correct answer for another reason too. B specifies "...to the control surfaces". There is no reason why specifically the control surfaces should or should not be damaged, and the rest of the structure not. In fact, my gut feeling tells me that even well above Va, hauling back on the controls will not damage the control surfaces, but will damage the main spar.

A is also not the correct answer since entering a zoom climb requires that you do not enter a stall. So hauling back fully is not going to work to enter a zoom climb.

My gut feeling is that actually either C or D is the correct answer, and will be along the lines of "the aircraft will enter an accelerated stall".

Anyway, what happens at Va, or one knot below or one knot above, is splitting hairs. Like others said, there is some measure of additional safety factor calculated in. So if you are a few knots above Va and you haul back on the controls, you will not break the aircraft. (But that's the kind of thing you should only do if you're a test pilot.)

[Andrewgr2]

And another thing (I believe)

At Va you can apply full deflection to any single control without risk of structural damage. However full up elevator and full aileron - or rudder - might break something ...

[Binners93]

Originally Posted by BackPacker

My gut feeling is that actually either C or D is the correct answer, and will be along the lines of "the aircraft will enter an accelerated stall".

Backpacker, I remember one of the answers was along the lines of 'The aircraft will climb at best rate/angle' and the other was something to do with ATC??? Hence why i dismissed them

[Baikonour]

Above The Clouds wrote

There is a % safety buffer built in,

Backpacker wrote

there is some measure of additional safety factor calculated in

Is that really so?
Can you point towards a place in e.g. CS-23 where there is a requirement for margin beyond, for instance, Va?

The reason for asking is that although we are used to most things being over-engineered, when there is no requirement to do so, we really should not rely on it actually being done...

B.

[keith williams]

Many pilots would define Va as something along the lines of " The speed at which the aircraft will stall at its limiting load factor" or " The maximum speed at which full control deflection can be carried out without exceeding the limiting load factor". But neither of these definitions are correct.

The following material is taken from a FAA discussions following the loss of the A300 fin over new York.

Quote:
Code of Federal Regulations Sec. 23.1507Part 23 AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES

Subpart G--Operating Limitations and Information
Sec. 23.1507[Operating] maneuvering speed.[The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than established in Sec. 23.335(c).]Amdt. 23-45, Eff. 09/07/93 Comments Document HistoryNotice of
Proposed Rulemaking Actions:Notice of Proposed Rulemaking. Notice No. 90-18; Issued on 06/15/90.Final Rule Actions:Final Rule. Docket No. 26269; Issued on 07/28/93.



The following text is extracted from the above document.

Reference: Conference proposals 119 and 120.
No action is being taken to amend Section 23.335 Design airspeeds.

Quote:
Explanation: Conference proposal 187 recommends revision of Section 23.335(c) to increase the design load factor to account for possible overloads resulting from maximum airplane maneuvers at speeds greater than VS√n for cases where the applicant chooses a design maneuvering speed greater than V=VS√n as allowed by Section 23.335(c). In support of conference proposal 187, the submitter states that the purpose of maneuvering speed (in addition to supplying a speed for design of control surfaces in accordance with Sections 23.423, 23.441 and 23.445) is to provide an operating speed where a pilot can be assured of not exceeding the design limits during maneuvers. If a design maneuvering speed in excess of VS√n is chosen (as currently allowed by Section 23.335(c)), and if the airplane is operated at that speed during maneuver, the potential exists for a pilot to exceed the design limit load factor unless that load factor is increased accordingly.

Post conference review indicates that the design maneuvering speed criteria provided in Section 23.335 is necessary and sufficient for control surface design. As such, design maneuvering speed selections greater than VS√n are appropriate, and requiring increases in load factor above those specified in Section 23.337 are unjustified.

However, the FAA recognizes that maneuvering speed is also used by the pilot as that airspeed below which full control surface inputs can be accomplished without structural damage. Maneuvering speed may also be used as a gust penetration speed to minimize the possibility of airframe damage. If the airplane is maneuvered at its maximum weight at airspeeds less than VS√n the airplane will stall prior to exceeding the maximum design load factor. If the airplane is operated at speeds greater than VS√n in the same conditions, the maximum design load factor can be exceeded.

So a single full deflection of the pitch control at exactly Va will not damage flight controls, but it may damage the aircraft structure, if the aircraft designers have selected a value of Va that is higher than the minimum permitted.

So option b in the OP's question may or may not be correct.

More discussion of the subject can be found at the link below.

http://forums.jetcareers.com/threads...-to-you.94018/

[ChickenHouse]

My first reflex was to take "PoF" as the sound you make upon impact doing so ... ;-).

[Mach Jump]

This is a very badly worded question, if reproduced accurately.

The Manoeuvering Speed (Va) is conventionally understood to be the maximum speed at which you can apply full 'up' elevator without exceeding the aircraft's maximum certified 'G' (wing loading), as the wings will stall and lose lift first.

If the aircraft is flying at exactly Va for its mass, and full 'up' elevator is applied, bearing in mind that it takes a certain amount of time, however short, to move the control to full deflection, the 'first thing' that will happen, is that during the short time taken to apply the deflection, the aircraft will begin to enter a 'zoom climb' with increasing 'G'.

As the elevator reaches full deflection, the aircraft should simultaneously reach it's maximum certified 'G', and stall.

Notwithstanding Keith's reference above to full deflection above VS\/n, but below Va, the aircraft should not be damaged, as a considerable safetey margin (usually +50% for aerobatic certification) is allowed above the maximum certified 'G' before permanent distortion to the airframe is sustained.




MJ