Instrument knowledge test

[mxalix258]

I'm getting hung up on this question (or variations of this question).

When flying a 3 degree glideslope, which conditions would a pilot expect concerning airspeed, pitch attitude and altitude when encountering windshear where a tailwind shears to a calm or headwind?

Answer:
Pitch Attitude: Increase
Required thrust: Reduced, then increased
Vertical speed: Decreases, then increases
Airspeed: Increases, then decreases
Reaction: reduce power initially, then increase

I can't seem to reason through what their thought process is on this problem, does anyone have any advice? I keep thinking in terms of the airplanes ground speed, and seem to get the opposite answer then the correct one everytime.

Thanks!

 

[EdFred]

I'm getting hung up on this question (or variations of this question).

When flying a 3 degree glideslope, which conditions would a pilot expect concerning airspeed, pitch attitude and altitude when encountering windshear where a tailwind shears to a calm or headwind?

Answer:
Pitch Attitude: Increase
Required thrust: Reduced, then increased
Vertical speed: Decreases, then increases
Airspeed: Increases, then decreases
Reaction: reduce power initially, then increase

I can't seem to reason through what their thought process is on this problem, does anyone have any advice? I keep thinking in terms of the airplanes ground speed, and seem to get the opposite answer then the correct one everytime.

Thanks!

Say you have an airspeed of 100 knots, and a tailwind of 20 knots. So you are moving across the ground at 120 knots. When you lose the tailwind suddenly, you still have enough intertia to continue moving at that 120kts into the new airmass. If the wind goes calm, you will now have an effective airspeed of 120kts. But the plane is trimmed for 100kts, so in order to maintain the 100kts of airspeed, the nose pitches up. So there's your Pitch Attitude answer.

Since you are temporarily going faster through the new airmass, you don't need as much thrust to maintain altitude. As it equalizes, it will be back to "normal."

Because your attitude changed with the transition between airmasses, your descent rate will be temporarily dampered. If you were descending at 700fpm, you may now only be descending at 400fpm. As things equalize, you will return to that 700fpm descent.

Because you transistioned the airmasses, and you had enough inertia to continue that 120kt ground speed when you punched into the new airmass, it would show your airspeed increase, until drag catches up and returns you back to 100kts indicated.

In order to bring the nose back down, you reduce the throttle, but you will need to bring it back to where it was before in order to maintain your previous pitch attitude and airspeed.

 

[dtuuri]

I keep thinking in terms of the airplanes ground speed, and seem to get the opposite answer then the correct one everytime.

Think of the shear as a gust. If the tailwind goes calm it's the same net result as a gust from ahead. The increased speed balloons the nose up and away from the glide slope. You'll mitigate that tendancy by reducing power and/or lowering pitch. After the plane restabilizes, it'll take more power to keep from descending below the glide slope at the slower ground speed.

See page 10-25 in the Instrument Flying Handbook.

dtuuri

 

[mxalix258]

ah...both those explanations make sense. Thank you very much! For some reason in my head, I always think as changes being instant, while in reality there is a gradual change as the airplane equalizes in the new airmass.