Originally Posted by
galdian
Yes, fair enough.
The question I am asking - at what height will you be taking the time to consult manuals etc to do the same.
800' AGL - my companies acceleration height?
1500'AGL - another standard used by many?
XXXX AGL?
I have no qualms doing the NNC, just a matter of when, in many cases suggest it can be delayed whislt everything's going OK with 15 degrees/MCT to MSA as an example.
Retracting flaps (if required/desired) - that's the next stage for consideration.
Cheers.
IMHO, two of the primary reasons that 737 pilots have so many questions/concerns about the Airspeed Unreliable NNC in the takeoff phase is 1) lack of detailed guidance from Boeing and 2) lack of specific training in the sim. I believe that this malfunction has been seriously neglected because it doesn't happen that often and there is an underlying presumption that the "bad" airspeed will be quickly identified without going through the whole checklist. However, there is no guarantee that
any of the airspeed indicators will be accurate. One of the possible scenarios is a bird strike that severely damages the radome (and possibly pitot/AOA probes) which then changes the airflow around the nose. In this case, every airspeed sensor is potentially effected.
I'll say again, this can be a very challenging procedure, and the crew does not have the luxury of figuring these things out on the fly. We really need to treat this as seriously as an takeoff engine failure and know (and have hopefully practiced) what we are going to do and when we are going to do it long before it happens.
Based on the scanty guidance, this is my recommendation on how to execute the existing Airspeed Unreliable NNC during takeoff.
- Identify and confirm. The "IAS Disagree" annunciation is the most responsive indication (difference greater than 5 knots for greater than 5 seconds). A single stick shaker is the most obvious (and most distracting). There is absolutely no need to do anything but establish and confirm a reasonable takeoff pitch attitude and power setting until 400'.
- At 400', if airspeed disagree is confirmed, announce "Airspeed Unreliable" and execute memory items. Declare an emergency. Prepare to execute engine out routing if applicable for the airfield. If you think about it, have PM check Probe Heat and grab QRH. Don't do anything else until your flap retraction altitude.
- Flap retraction altitude. Decide if you need to retract some flaps. If flaps are greater than 5 degrees, retract them to 5. If at 5 or 1 degree, leave them. Personally, I would only fully retract the flaps if I needed it for climb performance (some guidance from Boeing would be helpful here). Otherwise, I would leave them extended until at a safe altitude. If you need to retract the flaps, the PM looks up the appropriate climb pitch and power settings from the tables in the NNC. Set those parameters, retract the flaps, climb to a safe altitude.
- Once at a safe altitude, work through the rest of the checklist. If there are no reliable airspeed indicators by completion of the NNC, then plan to land at nearest suitable airfield with good VMC conditions with emphasis on trying to maintaining VMC enroute.
If the unreliable airspeed is generating a stall indication, be aware of the related system effects - Stick Shaker, Speed Trim Stall ID, Elevator Feel Shift, and Autoslats. Each of these will create an abnormal indication that can be mistaken for another malfunction. I would recommend knowing the location the stick shaker CB's (P18-2, P6-1) if the Captain wishes to exercise his emergency authority to silence this noise maker. As a side note, it is my current understanding that the 737NG simulator does not accurate replicate all of these ancillary system effects. I hope to confirm this my next training cycle.
If the unreliable airspeed is generating an overspeed, you will have an overspeed clacker. There is no way to silence this alert short of shutting off the bad side ADIRU, and you probably don't want to do that. If the
left side is faulty, that may trigger the flap load relief function (flaps will not extend fully). This function varies by aircraft, so check your FCOM for detail. Load relief only retracts flaps to the next higher setting, so you can still get to Flaps 30 by selecting Flaps 40. Depending on how far off the airspeed is, you may still be able to find a usable landing flap setting (Flaps 15) that does not cause the overspeed clacker to come on.
As I said, a lot of moving parts to this one. It is unfortunate it has not been given the attention that it deserves.