B737 FMA turns ARM when @ 800AFE

[tae9141]

Does anyone know why FMA turns ARM @ 800AFE when the plane is climbing out?

I looked up FCOM and the explanation is not clear why FMA will turn ARM so that the pilot can manipulate thrust lever manually. Also the magic behind why Boeing chose 800ft for the FMA to turn into ARM?

If you could post here a reliable link or reference, that will great.

Thanks.

[poldek77]

Originally Posted by tae9141

Does anyone know why FMA turns ARM @ 800AFE when the plane is climbing out?

I looked up FCOM and the explanation is not clear why FMA will turn ARM so that the pilot can manipulate thrust lever manually. Also the magic behind why Boeing chose 800ft for the FMA to turn into ARM?

If you could post here a reliable link or reference, that will great.

Thanks.

There is a paragraph in my company's OM-B Chapter 12-4 Automatic Flight:
automatic reduction to climb thrust occurs upon reaching the selected thrust reduction altitude which is shown on the FMC CDU TAKEOFF REF page 2/2 during preflight, or when the airplane levels off in ALT HOLD or VNAV PTH. Pilot entries can be made to override the default value. Allowable entries are 800 feet to 9999 feet.

I have found such a document: https://www.faa.gov/documentLibrary/...ar/ac91-53.pdf
Probably following this recommendation Boeing implemented 800 ft as the minimum thrust reduction height.
Standing by to be corrected.

[Tomaski]

It doesn't work that way on our fleet. How is your AFDS panel set up for takeoff? I'm assuming we're talking about a 737NG. If it is a previous model, I can't help you.

[Aoyb]

FCOM

Obviously the FCOM chapter 4.20.9 is the reference you’ve looked at.

800’ I suspect would be for the standard ICAO noise abatement profiles, which call for thrust reduction at 800’ (see ICAO doc 8168).

Having the auto throttle come out of THR HLD mode allows for the automatic setting of a climb power setting, or failing that, keeps the targeted TO N1 until the thrust reduction altitude entered in the CDU.
The FCOM (4.20.10) says that once the A/T annunciation changes from THR HLD to ARM, a reduction to climb thrust can be made manually by pushing N1 on the MCP.

[Skyjob]

THR HLD is engaged during departure until minimum THR RED height.
Some aircraft operators set the THR RED to 1500' AFE (2 engines), the absolute minimum is 800' AFE (any case).
When the minimum of 800' AFE is reached, A/T defaults to ARM until FMC commands A/T to set N1 and adjusts it to CLB thrust (provided a normal departure is flown).

The reason FMA is ARM is for enabling EOP to be flown with manual thrust intervention during procedure (which includes A/T disengage to prevent above mentioned thrust lever movement).

[Tomaski]

Originally Posted by Skyjob

THR HLD is engaged during departure until minimum THR RED height.
Some aircraft operators set the THR RED to 1500' AFE (2 engines), the absolute minimum is 800' AFE (any case).

Our fleet must be set at the higher value. Procedurally we select a climb mode (usually VNAV) at 1000 AFE, so the FMA never goes to ARM.

[Too Few Stripes]

The FMA changes to ARM at 800’ on NG series and 400’ on CL series. The logic allows the auto throttle to react to mode changes eg N1 or indeed to a second press of TOGA - below this height a second TOGA press will only move the thrust limit bugs to display the full thrust limit with no corresponding change in actual thrust, once FMA changes from THR HOLD to ARM a second press of TOGA will advance the thrust levers to full thrust.

[Too Few Stripes]

Technical Manual, Automatic Flight, Takeoff & Climb (this is from an -800 Manual)