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-   -   Flap and slat lock (https://www.pprune.org/tech-log/593616-flap-slat-lock.html)

applecrumble 17th Apr 2017 17:10

Flap and slat lock
 
Heya guys,
So let's say we have a flap or slat lock. Or both.
The pitch attitude on approach is higher (increased angle of attack) and we fly at a higher Approach speed.
If we fly a standard ILS without any failures, then we come down at 700FPM 2.5 degrees for a standard jet transport approach.
With the failure of the high lift devices we fly faster and to stay on the fixed glide path then we select a higher rate of descent to maintain the glide with a high pitch attitude.
My question is, why do we require less flare to arrest the rate of descent. I am aware of the tail strike implications of this failure.
But the FCTM says you will require less flare (understandable for tailstrike risk) but surely to reduce the higher rate of descent then you need a larger pitch increment rather than a smaller one? (Forgetting any tail strike implications)
I'm sure there is something simple I'm missing, but I did get up at 3am so please forgive me!

vilas 17th Apr 2017 17:55


So let's say we have a flap or slat lock. Or both. The pitch attitude on approach is higher.
All three cannot be considered in the same breath. Slats increase the attitude and flaps reduce it and together we get about +4 degrees on approach, not 2.5 degrees. If you only have slats it will significantly increase the pitch on approach. Something like eight degrees and that's why we need a small flare because tail strike can occur at 11.7. If you have slat jam the attitude is not a problem. If both are jammed the speeds are initially Vref+60 then Vref+50. There is no high angle of attack involved the speed increment takes care of that. It's V square remember?

Goldenrivett 18th Apr 2017 11:29


but surely to reduce the higher rate of descent then you need a larger pitch increment rather than a smaller one?
No - but you do need to start the flare earlier.

If you were on a 3 degree glide slope and didn't reduce the speed during the flare, then you would need to increase pitch by 3 degrees to fly level - no matter what your ROD was before hand.

Superpilot 18th Apr 2017 16:48

Higher ROD/Speed = An aircraft carrying more ENERGY. You require less elevator input because the elevator is more sensitive due to the high(er) energy situation.

JABBARA 18th Apr 2017 18:04


...the elevator is more sensitive due to the high(er) energy situation
I guess this is true at Direct Law. At normal law this energy vs. aerodynamic response relation is controlled/compansated by Flight control Computers. Side stick input is only a demand.


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