Deterrent Buffet Definition

[leonard17F]

Hello All,

I would be very interested in getting your inputs on the notion of "deterrent buffet".
After reviewing the official docs (AC's, etc..), I still have a doubt regarding its definition: is buffet so strong that it signals that the aircraft cannot be flown again, or is buffet so strong that you cannot read the instruments ?
I also have found somewhere the notion of "Excessive buffet" for which you cannot read instruments any more and which seemed to be more severe than deterrent buffet….

And the corollary of my question is:"can you officially define stall at DETERRENT buffet" ?

Thanks a lot,
Leo.

[CharlieJuliet]

Deterrent buffeting should, as its name infers, deter you from trying to pull any harder or fly any slower. I would assess deterrent buffet as a motion (usually a vertical bounce) harsh enough to cause discomfort to the crew which usually makes reading the instruments difficult (or impossible!). The Hunter T7 displayed a harsh vertical bounce which could be called deterrent buffet whereas a PA 28 just displays gentle buffet. To get the aircraft to stall the aircraft has to be flown through the buffet until a defined stall occurs. I no longer have access to Def Stans, but one definition of a stall is an uncontrollable nose drop - I am sure there will be others who can offer other definitions.

[Genghis the Engineer]

Is there actually any value in defining deterrent buffet formally and generically?

Surely this is an area where TP opinion should be paramount - defining and deciding what is adequately deterrent. That should surely include aspects of the aircraft role and ergonomics, plus task assessment as some tasks will take up greater aircrew attention than others, and hence need a much more obvious and obtrusive form of warning symptoms, such as pre-stall buffet.

But surely in all cases, deterrent buffet is about deterring a pilot from taking the aircraft further into an unsafe region. To use your extreme example of an aircraft being physically damaged, well I'd argue that if that was reached, then whatever deterrent effect was there, it wasn't deterring enough!

G

[FCeng84]

I think the discussion of Deterrent Buffet will benefit from a bit of background regarding what is it that is to be deterred - namely stall. Another concept that must be part of the mix is Stall Identification - i.e., characteristics of the airplane response that the crew will recognize as indicating approach to or onset of stall and thus prompt pilot action to back away from that condition.

First of all, the definition of stall. Normally with a given wing configuration (i.e., flap and speedbrake setting) increasing angle-of-attack will result in increased lift. The slope of the lift curve (plot of CL vs. Alpha) is the key aerodynamic parameter, CL-alpha. For most wings the lift curve will exhibit positive CL-alpha up to the AOA where significant flow separation occurs. AOA for CLmax is the angle of attach for which lift is maximum. Beyond that point further increase in AOA will not result in increased lift. For wings that exhibit a sharp stall break lift will actually decrease rapidly as AOA increases beyond AOA for CLmax.
In conjunction with the variation in lift that results from increasing AOA, the variation in pitching moment must also be considered. A wing that has positive pitch stability will generate more and more nose down pitching moment as AOA increases. This requires more and more nose up command to achieve flight at higher and higher AOA. For a wing with this charateristic, removing the nose up control input results in nose down pitch returning the wing to lower AOA. Some wings (particularly at higher AOA approaching or exceeding AOA for CLmax) will exhibit negative pitch stability whereby their tendency is to pitch up to even higher AOA as a result of an AOA increase. For a wing with this tendency, significant nose down control power may be needed to return from high AOA to low AOA.
A third consideration regarding high AOA is drag which generally increases with higher AOA. Of key concern is the ratio of lift over drag that is an indication of wing performance. At high AOA lift decreases while drag increases resulting in lower L/D and thus lower performance.

Now the question: What it is that needs to be deterred with regard to flight at or beyond stall AOA (i.e., AOA for CLmax)? Two considerations here. First is the loss of wing performance as measured by lift or the ratio of lift-over-drag. Second is the prospect of pitch-up instability and potential loss of pitch control if the aerodynamic pitch up is strong enough to overwhelm the available nose down pitch control power. (This is often referred to as "locked in stall".) The first of these is not desirable and, if close enough to the ground, is a real threat to continued safe flight and landing. The second is a potential short term disaster as it may result in loss of controlled flight.

In order to avoid the pitfalls of stall the FAA regulations call for airplane response characteristics that give the flight crew positive indication of the onset of stall. There are three means of providing Stall ID:
1. Positive nose down pitch break that cannot be arrested by the pilot. In this situation the airplane naturally recovers from high AOA by itself.
2. Deterrent Buffet - the airplane exhibits sufficient buffet that the flight crew will be deterred from commanding even higher AOA.
3. Full nose up pitch control input from the pilot with no further increase in pitch attitude. The airplane will not go to extreme AOA where either significant loss of performance or pitch stability may occur.

As a closing thought it must be recognized and remembered that there is a very wide range of airplane response characteristics at high AOA. For some wings approaching AOA for CLmax will be very smooth and controlled. For others it will involve lots of buffet. The most concerning situation is an airplane that exhibits a sharp pitch-up instability without much buffet to alert the crew as AOA increases. If an airplane does have a significant pitch up at high AOA that risks locked in stall there must be provisions built into the system to either prevent the pilot from go to that level of AOA or there must be sufficient stall buffet to deter the crew from commanding the airplane into that dangerous situation.

[leonard17F]

Dear FCeng84,

Thanks a million for your very thorough and pertinent reply.

I will digest it over the week-end and try to response ASAP.

Leo.

[zzuf]

From my experience at "deterrrent buffet levels" there is zero incentive to fly any slower. Severe vertical and lateral shaking of the flight deck would be typical.
Clearly this is a qualitative assessment made by experienced TP's.
Also just because the stall has been defined in this way it doesn't necessarily mean that "dangerous characteristics" will be encountered at higher alpha.
Surprisingly deterrent buffet levels may not necessarily be a structural consideration.
One type I flew could suffer structural damage to the stab with no stall buffet noticeable to the crew. The stab was always instrumented for structural loads prior to stall tests. This was despite the flaps up stall being defined by deterrent buffet.
It would seem that deterrent buffet levels are pretty well always a stall handling issue - I personally never encountered it being an issue during stall speed determination. Would be an interesting horse trading exercise to argue deterrent buffet verses the need to achieve a particular stall speed.
Stall buffet levels seem to be typically low in the configurations where stall speed is an important performance consideration.