Engine Behaviour
 

I would like to know how does the auto-throttle behave in a strong headwind variation, i.e. does it change the thrust values to keep the airspeed constant or remains unchanged during that time? Is there any type of compensation during an approach, or one must disconnect the a/t if this kind of situation occurs?

GD&L

The airspeed is not affected in any way by having a headwind. The groundspeed is though... but I have yet to see any flight deck automation which will try to follow a set groundspeed. IAS is the target variable set.

If there are such systems out there, I'd be very interested in hearing about them though!

Cheers,
Fred

It's not about having a headwind, it's about having a sudden _change_ in headwind, and this does affect your airspeed.

As far as I know, all autothrottle systems are able to cope with these changes and adjust the throttle setting to match the IAS set on the MCP when a change in airspeed is felt. ;)

This may lead to your throttles "chasing" the IAS when you have some turbulence, in which case it would be a better idea to turn the A/T off to keep it from wearing out the all the bits and bobs involved.

There is one occasion when the wind can affect the A/T, which is on the takeoff ground roll.

The A/T goes into THR HOLD at 80 kts g/s. If you get a sudden strong gust of wind, the A/T might well go into THR HOLD early, leaving you to do the rest!

Actually all FBW Airbuses have a system that regulates the thrust via the autothrottle to maintain a minumum groundspeed on approach.

The system is active when the approach is activated and varies the speed target wrt the headwind being sensed compared with the wind inserted into the approach page of the FMGC from the ATIS.

In very strong headwinds therefore the G/S Mini will advance up the speed scale and the autothrust will respond to maintain the minimum groundspeed and thus maintain not only a stabilised vertical descent rate but also a minimum energy level in the a/c should the headwind suddenly die away.

Clever huh?:ok:

A-FLOOR,
d'oh! Thanks. :O

Thank you all. I did imagine that the a/t would somehow compensate for the sudden changes in headwind strenght.

As all automation processes imitate (liberate?) the pilot's repetitive work, I am wondering how does a pilot flying a classic a/c ( 727 comes to mind ) cope with the exact same situation. Would anyone care to share that experience?

Geardownandlocked

The average pilot has his senses mightily aroused and the adrenalin pumping in conditions of heavy variablw wind shear. He uses the little levers bunched in his clammy right hand to their maximum if he has to and occasionally will wish he could go through the stops. If it gets to be too demanding he will thumb his nose at the Airline Bean Counters and go some place else.

A good A/T design should not suffer too many problems from wind or gusts. Some designs incorporate angle of attack or angle rate stabilisation; others input ground speed or may have a complicated energy computation.

Capt S, which system use 80 kt GS? All of the A/Ts that I have seen all used IAS for the THR HOLD function.

SafetyP - my apologies - I meant 80 kts IAS. G/S would make a total nonsense of the rest of my post.