Single engine driftdown.

[Foxcow]

If anyone could offer any insight, I would be grateful. Lets say you have two aircraft that are the same type, weight, and configuration. One is cruising at FL350 the other at FL310. My groundschool instructor explained that if they lost an engine at the same time, they will both reach their single engine driftdown altitude at the same time because:

-the aircraft at the higher altitude is cruising at a lower IAS so it will reach its initial driftdown speed faster and start descending sooner.
-the aircraft at the higher altitude will also descend faster for obvious reasons.


This almost makes sense but it seems like there is more to the story. I would think the lower aircraft will get down to altitude before the higher one.

 

[BobDDuck]

They will get to the base altitude at the same time but the higher aircraft will have covered more ground doing it.

 

[seagull]

If anyone could offer any insight, I would be grateful. Lets say you have two aircraft that are the same type, weight, and configuration. One is cruising at FL350 the other at FL310. My groundschool instructor explained that if they lost an engine at the same time, they will both reach their single engine driftdown altitude at the same time because:

-the aircraft at the higher altitude is cruising at a lower IAS so it will reach its initial driftdown speed faster and start descending sooner.
-the aircraft at the higher altitude will also descend faster for obvious reasons.


This almost makes sense but it seems like there is more to the story. I would think the lower aircraft will get down to altitude before the higher one.

I would have to look at the performance, maybe I'll play with it on the computer tomorrow, however, on face value, I don't think I agree with your ground instructor for a variety of reasons.

 

[B767Driver]

My performance charts do not include the rate of descent during DD. However, my experience in the simulator is that the DD rate of descent is very low....just hundreds of feet per minute. The airplane at FL310 will decelerate pretty quickly to DD speed. Let's just say within one minute.

The airplane at FL350 would have to descend at 4000fpm to reach FL310 and begin it's descent simultaneously with the FL310 guy. Then they would descend identically to DD altitude.

It doesn't seem logical to me. It could be true. I'll be interested to hear back from those with more detailed data.

 

[NJA_Capt]

My ground school instructor explained that if they lost an engine at the same time, they will both reach their single engine driftdown altitude at the same time.

Maybe for some airplanes.

For the Citation X:
Driftdown from FL350 will take 52 minutes and cover 281 nm.
Driftdown from FL310 will take 42 minutes and cover 223 nm.
Roughly 300fpm for each a/c to reach 18,500' at max weight.

Consider this: At low weight and ISA temps at FL310, we would have to climb to the SE enroute altitude.

Just for grins: It would take 27 minutes to drift down from FL490 to FL338 (SE enroute alt).

A higher aircraft may be closer to the DD IAS, but there is less wind resistance to help slow it down. To be honest, there isn't much difference in FL310 and FL350.