Camel asked the following questions:
Q) How low can you go on Autopilot on a non precision approach?
A) It very much depends on which of those first three modes (
which I described above) that you have selected, e.g. :
Mode 1) If aircraft is descended via vertical speed mode and the MCP altitude window is set to MDA (
usually rounded to closest 100 ft above), the autopilot will level off at that altitude defined on the MCP:
OR, if the aircraft is descended via vertical speed mode and the MCP altitude window is set to the Missed Approach Altitude, the autopilot will descend the aircraft until it impacts the ground (
or ocean).
Mode 2) If the aircraft is descended via VNAV PATH the autopilot will stop the descent at whichever level is defined as the lowest level in the legs page of the FMC for the selected approach.
Mode 3) If the aircraft is descended via IAN, the autopilot will follow the vertical profile exactly as if the approach was an ILS (
down to about 50' ?), and / or I suspect there will come a point when it might drop back into Control Wheel Steering (CWS).. though I've never investigated just what are the limits on this (
company procedures prohibit it... though perhaps it's one to try in the sim?!)
Q) If you should disconnect the A/P at very low level, but leave the autothrust engaged , what would happen if you pitch up to go around and push the power levers forward,but dont press the TOGA buttons?
A) You will probably have a speed set in the MCP speed window, i.e. a speed appropriate to the flap setting. With the auto thrust engaged (
regardless of whether you descending, or flying level, or climbing) the auto thrust system will do its best to maintain that MCP speed. Indeed you wouldn't actually need to push the power levers forward yourself, as the AutoThrust system will do this for you, i.e. to maintain the MCP speed.
Nb. In the scenario you describe, the aircraft / FMA will still think it's in the selected approach mode (
because you haven't told it otherwise, e.g. by pressing the 'go-around' button, or re-selecting another mode on the MCP) and as such the flight directors will command a nose down pitch, i.e. to try an get you to follow the vertical profile of the descent (
from which you are now rapidly diverging in a climb).
I should add (
hence the edit) the due to the potentially bogus guidance that the Flight Director system can give you, i.e. when descending below the MDA on a non-precision approach, it is customary (
and good practice) on the B737 - once you are visual with the runway (
and certainly if continuing below the MDA) - to turn both Flight Directors briefly off and then on again. The causes the Flight Directors to disappear out of view for the remainder of the approach to the runway (
i.e. you continue the landing entirely 'visually'), however, should you press the TOGA button, the Flight Directors will then reappear to provide go-around guidance. This is typically known as 'Flight Director Pop-Up Mode'