B737 Minimum speed reversion modes

[airamerica]

B737-700

Can anyone be so kind as to explain what the subsequent modes will be entered when say the autothrottle is disconnected and the aircraft reaches minimum speed and continues to slow down?
AFDS pitches nose down, then what, keeps descending until when?
The same in the climb, if the aircraft cannot maintain the minimum rate of climb for the given speed, the ap disengages and the fd’s retract, then what?
What will the ap revert to say in pitch for eg?
The volume 2 gives a too vague explanation on minimum speed reversion and subsequent modes of operation if the pilot does nothing.
Cheers
(Yea I know that I should go out a bit more often)

[alexban]

In case of minimum speed reversion,the AFDs reverts to LVL CHG,and it command a speed 5 kt greater tahn minimum speed.Reaching this speed the normal MCP speed selected is reactivated.The AFDs will maintain this speed in LVL CHG.
In case of minimum ROC,with minimum speed and LVL chg,and speed is decreasing,the a/p disengages,f/d retract,and you are in command..manually.This is the case when you want to climb much higher than the max cruise in the FMC..why would you do that? The a/p 'assumes' you are doing a test flight,so..:=

[blackmail]

hello air america & others,

to follow on on alexban, i would say: use the proven "kiss"(keep it simple st*pid") principle & think basic flying : if you where the autopilot, how would you handle/correct the situation?

basically you have overspeed/placard(gear & flap) protection & underspeed protection . an additional performance reversion mode exist for placard speed & minimum speed.

1) if all the gimmicks: autopilot/flightdirector & autothrottle are engaged & all is functioning normally, you will have full speed/reversion mode protection :

- assume level flight for simplicity, if tending to fly to fast, the autothrottles(a/t) will reduce thrust & keep slightly below limit speed.
- same , if too slow, a/t will increase thrust & keep slightly above min speed.
a speed >15kt must be selected to remove the underspeed symbol in the mcp window.
- let's climb, we now have an additional protection : min speed reversion mode: if asking an excessive performance in vertical climb speed(v/s), when reaching a limiting minimum indicated airspeed & a/t condition: autopilot reverts to level change & controls pitch with attitude.
if then a minimum performance level (v/s), cannot be maintained without decelerating, the a/p disconnects & it is up to you to take over control.

2) in your example, autothrottle disengaged, you still have limited speed protection.
- vmo/mmo protection : yes(a/p goes in level change).
- placard protection for gear & flap : no.
- performance reversion mode for min speed : no. so in this case you can stall the aeroplane when in alt hold or glide slope capture,(please, don't).
as a gop(good operating practice) : if flying on automatics : use a/p + f/d + a/t, if flying manually, use hotas : hands on throttle & stick except for take off & climb, where you can use auto throttle (cfr mr boeing recommendation).


for info, speed window on mcp(mode control panel), will flash in above modes preceeded by overspeed(filled 8 symbol)/underspeed(capital A symbol).

a last remark & i hope many will agree, the interphase man(or woman)/machine & logic of the boeing automatics are not so difficult to master & understand, but the accuracy of keeping selected parameters by the autopilot would definitely need some additional engineering, to keep it mild.

hope this helps,
kind regards,
bm