The TNT B737 EMA/Birmingham incident thread

[captjns]

There is too much to speculate upon here without full details.

The second A/P must be engaged in CMD by 800 feet RA to execute a dual channel A/P approach. Otherwise, CMD engagement of the second A/P is inhibited.

The A/P automatically disengages when any of the following occurs:

• pushing either A/P disengage switch
• pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
• engaged in CWS or CMD above 2000 feet RA with flaps not up or G/S engaged.
• pushing either TO/GA switch after touchdown with both A/Ps engaged in CMD
• pushing an illuminated A/P ENGAGE switch
• pushing the A/P DISENGAGE bar down
• activating either pilot’s control wheel trim switch
• moving the STAB TRIM AUTOPILOT cutout switch to CUTOUT
• either left or right IRS system failure or FAULT light illuminated
• loss of electrical power or a sensor input which prevents proper operation of the engaged A/P and mode
• loss of respective hydraulic system pressure.

There is the possibility that late in the approach it was discovered that the second A/P was not in CMD because there was no “Flare Armed” observed on the A/P Mode Display”. Then the operating A/P was inadvertently disengaged by pushing the illuminated switch which would thus cause the A/P to disengage. I have observed this while conducting simulator training.

A successful auto-land can be accomplished, even if the second A/P is engaged below 800’, provided the aircraft is established on the localizer and glide slope centerlines with minor deviations and the flight control computers have enough time to perform their self tests to ensure the integrity of the auto-land system.

Translation of the DFDR will clear up any speculation regarding the A/P issue. Either way it’s a crappy situation to be in. The best news is that nobody went to the hospital.