View Full Version : ALT HOLD & climbing in VNAV

10th Jul 2001, 03:45
...in a Boeing that is.

If climbing with VNAV engaged, to say FL300, and with MCP ALT set to 30000 for instance, engaging ALT HOLD on the MCP would cause the auto pilot to command a level off at an altitude marginally higher (say 28000 if the hold was pressed at 27500 and according to rate of climb) than when the mode was selected and according to a 'smooth' transition from climbing attitude to level flight... Am I right or wrong?


10th Jul 2001, 07:27
Given your scenario, in 737-300 hitting the ALT HOLD button will cause the aircraft to nose-over and descend to the ADC altitude where the ALT HOLD button was pushed. Because the MCP altitude differs from ALT HOLD altitude there is no ALT ACQ anunciation. So if you hit ALT HOLD right on 28000 ft the AP will level off and smoothley descend to that level.

Bally Heck
10th Jul 2001, 14:37
The trick is to press alt hold twice. Using your skill and judgement press it say 500 to 1000 ft before your required level off alt. Then press it again when the v/s is zero. That way it wont descend to recapture the original alt. VNAV or alt cap are much easier to use though. :cool:

10th Jul 2001, 15:11
The same, double tap, method can be used to level off during a coupled non-precision approach at MDA. 10% of the ROC/D as anticipation seems to work for me.

10th Jul 2001, 15:52
I seem to remember in the 757 if you pressed the alt hold once it would level you off slowly (relatively smoothly!) but if you double press it, it would take you back to the altitude you were at when you first pressed the alt hold.

But it was a long time ago!

10th Jul 2001, 19:30
According to the 747-400 Manual, the aircraft should return to the altitude at which you pressed the button.


10th Jul 2001, 22:31
Huh excuse me
Leveling off at MDA ?

Never heard that before , except in lightplanes !

And they don't have an ALT HOLD. :D

10th Jul 2001, 23:38
E.Morse...There is nothing wrong with flying level at MDA to the MAP...however operators configure their a/c in various ways for the non precision approach (NPA).This is what ultimately determines whether one can fly level to the MAP on a NPA.

Speedbrake UP
11th Jul 2001, 07:48
During Visual Circling, the double tap method works well for an MDA of eg. 730' Setting the MCP to 700' then pressing the the Alt hold firstly to slow the rate of descent and a second time to stop your descent at an altitude of 730'. As Razor stated 10% of your rate of descent seems to work well.