Vertical Mode Trap
Gulfstream G450 Abnormals
Figure: G450 Guidance Panel No Vertical Mode, from Eddie's aircraft.
Figure: New Orleans Lakefront ILS Runway 18, from Jeppesen Airway Manual, page KNEW 21-1, 13 Apr 12.
We were at 3,800 feet descending to 3,000 feet on an intercept to the Lakefront localizer when we were cleared to descend and maintain 1,700 feet until established.
The copilot dialed the altitude selector to 1,700 feet just as the autopilot captured 3,000 feet. I pointed to the new altitude on my pilot's flight display and acknowledged the new altitude clearance. It was a dark, moonless night and we were still 15 nautical miles north of the field. I asked the copilot if we were cleared the approach and he said no, just to intercept. In our discussion neither of us noticed the vertical mode of the autopilot was gone, though the airplane continued the previously programmed descent rate of 1,000 feet per minute.
The localizer was centered and the glide slope was full scale deflection high, as would be expected this far from the runway. I watched the airplane reach 1,700 feet as was directed in the guidance panel and was dismayed to see it continue its descent below our assigned altitude. At 1,500 feet I disengaged the autopilot and brought the airplane back to altitude.
The copilot heard the autopilot chime and asked, "what happened?" "I don't know," I said, "it missed the altitude."
I reengaged the autopilot and was happy to see it perform normally for the rest of the flight.
Figure: Side view of example approach, from Eddie's notes
We kept an eye on the automation to see if the problem would repeat itself. After three or four subsequent examples we figured out the autopilot's sequential thought process: (1) Altitude captured, vertical mode isn't needed so disengage it. (2) New altitude dialed in but no vertical mode, go to pitch hold mode.
That the G450 autopilot will do this is dismaying, to be sure. I'm not sure other aircraft have the same issue but the issue is there for all G450 operators.
Figure: Vertical Mode Technique, from Eddie's notes.
We've been doing this for a while now and just about every three months or so it goes like this:
- PM: (Changes altitude select knob) "Four thousand feet."
- PF: (Points to PFD) "I see four thousand, and . . ." (points to guidance panel) ". . . whoa, no vertical mode, setting one now."
At the very least the technique has saved a violation or two. I'd say it is worth adopting.
Jeppesen Airway Manuals