757 Flap Load relief Airspeed Data
Thread Starter
Join Date: Mar 2007
Location: south east UK
Posts: 375
Likes: 0
Received 0 Likes
on
0 Posts
757 Flap Load relief Airspeed Data
Does anyone know where the 757 flap load relief system gets its airspeed input from, and is it hard wired or do any of the alternate switch-over options change it?
I'm assuming it comes from the Left ADC or the Capt Pitot due to something I saw in the sim the other day, Its assumption but its my best guess.
Scenario.
Blocked pitot noticed on the climbout during accelleration - speed call from the left seat - my ASI looks fine - quickly notice that they do not agree - check the standby - it agrees with mine - completed accelleration / flap retraction check a few pitch power settings and fairly sure that FO and Stby ASI are OK, capts is overreading.
So far so good. Switching to Right Air data on the capts side sorts his instruments. Right Autopilot works ok
So off we bimble onto a LOC / DME aproach - Flap 30 is selected and only 25 is indicated, we're on the approach now - I decide its not the time to deal with it and ask for Flap25 to be gated as its an acceptable landing config - and I don't want Flap 30 to suddenly be delivered at a surprising time - like in the flare!
The only plausible reason we could come up with is that it was the flap load relief system preventing flap 30 from being delivered - The capts ASI was reading off the right ADC at the time -but the left ADC and pitot would still be giving duff data in the background
Just one of those little things that makes you wonder! Anyone got any better ideas or were we on the money with the relief system being hard wired to the left ADC? And is there anything else hard wired to any of the ADC's which could be little gotcha?
I'm assuming it comes from the Left ADC or the Capt Pitot due to something I saw in the sim the other day, Its assumption but its my best guess.
Scenario.
Blocked pitot noticed on the climbout during accelleration - speed call from the left seat - my ASI looks fine - quickly notice that they do not agree - check the standby - it agrees with mine - completed accelleration / flap retraction check a few pitch power settings and fairly sure that FO and Stby ASI are OK, capts is overreading.
So far so good. Switching to Right Air data on the capts side sorts his instruments. Right Autopilot works ok
So off we bimble onto a LOC / DME aproach - Flap 30 is selected and only 25 is indicated, we're on the approach now - I decide its not the time to deal with it and ask for Flap25 to be gated as its an acceptable landing config - and I don't want Flap 30 to suddenly be delivered at a surprising time - like in the flare!
The only plausible reason we could come up with is that it was the flap load relief system preventing flap 30 from being delivered - The capts ASI was reading off the right ADC at the time -but the left ADC and pitot would still be giving duff data in the background
Just one of those little things that makes you wonder! Anyone got any better ideas or were we on the money with the relief system being hard wired to the left ADC? And is there anything else hard wired to any of the ADC's which could be little gotcha?
Join Date: Dec 2006
Location: belgium
Posts: 236
Likes: 0
Received 0 Likes
on
0 Posts
From the maintenance training handbook:
FSEU -1 energizes the flap PDU control valve with left hydr pressure to retract the flaps at load relief conditions. Flaps go to 25° from 170kts, and return to 30° when getting back at 165kts. Flap lever does not move, only needle on indicator moves.
load relief is only available on hydraulic operation, not electric.
FSEU-1 gets airspeed data from the left air data computer. No ADC inputs are shown, on my diagram, to FSEU2 and 3 from ADC right.
Now this could indeed create situations when the left ADC is going bananas.
However, the text also mentions that ADC alternate switching is detected by the FSEU. It doesn't mention further if this is for load relief or other FSEU functions.
FSEU -1 energizes the flap PDU control valve with left hydr pressure to retract the flaps at load relief conditions. Flaps go to 25° from 170kts, and return to 30° when getting back at 165kts. Flap lever does not move, only needle on indicator moves.
load relief is only available on hydraulic operation, not electric.
FSEU-1 gets airspeed data from the left air data computer. No ADC inputs are shown, on my diagram, to FSEU2 and 3 from ADC right.
Now this could indeed create situations when the left ADC is going bananas.
However, the text also mentions that ADC alternate switching is detected by the FSEU. It doesn't mention further if this is for load relief or other FSEU functions.
Join Date: Dec 2006
Location: belgium
Posts: 236
Likes: 0
Received 0 Likes
on
0 Posts
So I'm no pilot, but only two solutions come to mind when the left ADC fails : or pull the CB of the load relief system; or select flaps electrically. Maybe the last one should be mentioned in flight crew procedures on adc fail?
