LVL CHG w/o AT armed

[Dimanski]

Good time of day 2 everybody!

I'm interesting in such question: we have B737 with AP engaged, but inoperative autothrottle. For climb we select LVL CHG on MCP, and set there ALT (for example of FL). With AT engaged AP will stop climbing on MCP ALT. But what will be in our case without AT? FMA will annuciate ALT HOLD mode, and speed will begin to grow, or AP will miss our MCP ALT and aircraft will climb futher?

AFS also limits our speeds, that it's not exceed Vmo and minimum speeds. Does it afford to speed grow to Vmo and then will switch AP to LVL CHG, or leave ALT HOLD?

So in a few words: AP actions on MCP ALT reaching with LVL CHG selected and A/T inoperative.

Big thankx!

[Grasscarp]

There is a thread going on about level change in Tech at the moment. Your question is possibly not on the best thread to get noticed.

[Capt Pit Bull]

The AP will capture the altitude. The speed will increase. The AP is no longer controlling speed, control to a speed limit is a function of the AT at this point. If its inoperative then there is no speed protection, the crew must manually select the required thrust. If they forget, the overspeed warning should give them a clue.

Pitbull's golden rule for operation without autothrottle:

IF THE PITCH MODE CHANGES YOU NEED TO MOVE THE THRUST LEVERS.

[Mr_ATPL]

You would have 2 scenarios with the LVL CHG function.

1) If the MCP ALT is set to a higher alt. The aircraft will pitch for speed selected in the MCP SPD windows. Now as the altititude is higher it will NEVER DESCENT, if you manually have set the power (saying that the AT is inop) your climb rate now depends on how high your AT (N1) is set. Once level, the LVL CHG will change to ALT HOLD and all of the excessive power set for the climb will now "change over" into excessive speed, which might EXCEED VMO/MMO, again all this pending on where your have left AT.


:confused:
Think about this way, driving in you own car, up a big hill in second gear. Now, after you reach the top of the hill and it starts to go downhill you need to let go of the accelerator in order not to exceeed the RPM (red limit).
:confused:



2) IF you set the MCP ALT to a lower altitude. The aircraft will pitch for speed selected in the MCP SPD windows. Now as the altititude is LOWER it will NEVER CLIMB, if you manually have set the power (saying that the AT is inop) your descent rate now depends on how high your AT (N1) is set. (The higher the power the LOWER the rate of descent) Once level, the LVL CHG will change to ALT HOLD and all of the IN-excessive power set for the descent will now "change over" into IN-excessive (low) speed, which might develop into a low speed situation - again all this is pending on where your have left AT.

Hope it all makes sense... If not post a query.

[Dimanski]

Grasscarp: I don't know to what theme this question is more applicable.

To others: Just wanted to know AFDS actions in this case, but thankx for such detailed answers. :ok: