View Full Version : B777 Unreliable Airspeed Checklist

22nd Nov 2018, 03:48
Was wondering in the B777 Unreliable Airspeed Checklist, why does the checklist ask us to use Alternate Flaps Extension?

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1780x2000/unreliable_airspeed_f5cf31ef008d78fbb83ddf518834e88e99713116 .png

22nd Nov 2018, 04:08
If the airspeed is unreliable, the FSEU cruise inhibit and load relief functions might not work correctly, which could affect slat/flap operation. Those functions are bypassed when the flaps are in ALTN mode.

22nd Nov 2018, 10:45
Thanks that makes sense.

Why do we also disconnect PFC if the airplane cannot be trimmed?

22nd Nov 2018, 11:09
Because you will be continually fighting the aeroplane in pitch if you leave the PFCs in. Far better to go into “direct” manipulation of the flying control surfaces at that point and use the natural longitudinal stability of the airframe to help.

22nd Nov 2018, 11:26
Interesting. On 747-400, extension of flaps is in normal mode until 20. Then alternate for 25, which is used for landing.

22nd Nov 2018, 11:46
Thanks that makes sense.

Why do we also disconnect PFC if the airplane cannot be trimmed?

In normal mode, pitch trim is achieved by changing the trim reference speed. The PFCs move the elevators to trim the aircraft and the stabiliser then moves to streamline the elevator. The PFCs need airspeed data to perform that function. If you can trim the aircraft, well and good. If not, then you must disconnect the PFCs and revert to direct mode to restore pitch trim. In direct mode, the pitch trim switches move the stabiliser directly.

27th Nov 2018, 04:33
Thanks for the excellent answers

5th Oct 2022, 12:15
How is it possible that the GS is still reliable if it comes from the ADIRU?