PPRuNe Forums - View Single Post - Brand new Boeing unreliable airspeed procedure
Old 8th Apr 2014, 02:50
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Becalmed
 
Join Date: Apr 2014
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Hi Bloggs,

The logic behind the Airpspeed Unreliable checklist is not only related to speed margins. Down low it is absolutely critical what you set because that's where the ground is. If I ignored the memory item in IMC at say 4000' AGL because I couldn't believe Boeing's numbers and set, say 2 degrees ANU and 80% N1 I'd be descending toward the terrain instead of getting away from it. No biggie at Singapore, for example, but maybe fatal at Hong Kong

I acknowledge that up high the choice of pitch attitude and thrust setting is more critical to stabilised flight because of the reduced margin between Vmin and Vmax, but the memory item is designed to place the aircraft in a safe state while you ferret around for the QRH or ECL and apply it. If you're cruising at FL350 or even climbing to it or descending from it in the flight levels, you're already safe. Touch nothing, get out the checklist and start solving the problem.

The B787 memory item is;

Autopilot Disconnect Switch................ PUSH
Both A/T Arm Switches.......................OFF
Flight Director Switches (both).............OFF
Set the Following Gear Up Pitch and Thrust:
Flaps Extended------------------ 10 degrees and 85% N1
Flaps Up------------------------- 4 degrees and 70% N1

Were I to detect Airspeed Unreliable in the cruise, I would call the memory item, action the first three and then verbalise to the FO that I won't touch the pitch and thrust as we're already in stable and level flight, well above terrain and ask him or her to access the Airspeed Unreliable checklist. To blindly apply the 4 degrees and 70% N1 at FL430 would certainly cause severe grief and this is where common sense steps in.

As for the B787 Airspeed Unreliable, another function of the pitch and thrust settings are to create a near-enough to stable state of flight. Once safely away from the ground (be it on departure having used 10/85 or in the cruise at FL430 having left the pitch and thrust unchanged), the crew can then use the flight state and checklist to identify the dodgy speed source.

Where Airbus has the Back Up Speed Scale (BUSS), the 787 has a function where speed calculated by GW, AoA, Config & GPS altitiude replaces indicated airspeed on the PFD: this is what Gas Path refers to & the point of it is to get the beast back on the ground while minimising pilot workload.

I'm really with Italia458 on this one. Boeing has designed this procedure because they know a lot more about it than we do and they want us to just do it, not to argue with it. Understanding the logic is great (get out your FCOM/QRH performance numbers and see what it gives you) but just keep it simple!
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