Autothrottle off or Arm on the B737

[Centaurus]

A search indicates that this subject has been discussed previously. The most recent Boeing published revision to the B737 FTM states that the use of ARM for landing is not recommended. This is the first time that the manufacturer has seen fit to make a policy statement on the subject.

This suggests that something undesirable can occur if the autothrottle is left at ARM and speed deselected. As usual Boeing are wary about explaining exactly why they recommend a specific handling policy - which goes part way to explain the plethora of different handling techniques espoused by different airlines for the same aircraft type.

Leaving the autothrottle at ARM and deselecting Speed is a common technique used by 737 operators, but I now wonder if this is wise, considering the latest Boeing words on the subject.

Request comments.

[BOAC]

Never been a great fan of 'Arm' for landing, as I have heard of and seen examples of the so-called 'alpha-floor' kicking-in during a flare at a higher than normal retard height (>25ft, from memory?), when it detects an ensuing 'stall', and application of reduced g/a power at that moment is disconcerting! It can also be a nuisance at some weights/flap settings since 1.3xVstall can be less than Vref+5 (not that anyone seems to fly that :*) - I have the 'calcs' somewhere.

Reasons given to me by a very boring IRE for using it were:-

Stall protection (on short finals!! :eek: )

Autothrottle g/a application (useful on s/eng g/a), thereby

Allowing use of reduced g/a power (one push of g/a button) rather than HAVING (his words!) to use full g/a to satisfy the 'rules'.

We'll have to wait for the 'revision' and see what happens?

[Bearcat]

BOAC, i have heard all these argueements before re having arm engaged. If Boeing are uneasy about it dump it. I have seen over the years different angles trying to operate the 737 3/4/5 and mainly these new ideas come from new operators/ charter outfits. Alot of charter outfits started to use N1 and a click at 1500agl so the bug would drive to plus 20. I would go into it here but by doing that one is leaving them selves hole open for many scenarios as against keeping it simple as per what boeing says by selecting what they recommend as against some new ideas by non boeing test pilots. Flying 737s is a fairly uncomplicated business that can turn into a can of worms when the test pilots get their paws involved.

Capt Bear

[purr]

certain nonnormal checklists like assy t e flaps could have you flying in the yellow band on approach . Having the A T on arm may not be a good idea in such situations.I think for visual approaches or even ILS/VOR keeping the AT to arm may not be such a bad idea .

[FoxLima]

I recall the info from Boeing:

"It is recommended to switch off the A/T as in the ARM-mode it may react to gusts during landing and the possibility of a tailstrike is more likely."

FL

[Menen]

The autothrottle should be completely switched off for a SE approach and SE go-around (says so in the Boeing FTM) - so the question of it being available for a SE go-around never arises.

Which raises a similar question. Why is it that the autothrottle is switched off as part of the recall actions associated with an engine fire drill at 400 feet after lift off - yet not switched off until the item is read as part of an engine failure (flame-out at V1)and shut-down drill which Boeing say should not be actioned until the aircraft has been cleaned up and max continuous set?

Seems to me that when an engine fails the auto-throttle should be disconnected immediately?

[Lump Jockey]

If you keep it at ARM, is it inactive then? Does it then only require the switch of the A/P to be turned to "on" to activate it? I assume the speed has already been dialled in at this stage? (Curious non-pilot here!) Is the AT operable ONLY with the A/P being ON, ie. it is coupled with the A/P, and can only be used witht the A/P? Or is it usable as a single operational tool? How often is A/T used, during which stages of flight for example?
Thanks again, LJ.:p

[timzsta]

ATPL student here - keen to learn as much as possible about the 737!

I thought the A/T is armed as part of the before take off checks, so that the autothrust sets the correct power for take off. Once airborne and vertical path/speed modes are selected ie IAS/VNAV the A/T then controls power/pitch to achieve desired speed/ROC etc.

On finals the A/T is selected off and speed controlled manually. In event of N1 exceeding a preset value (is it about 85%) the auto thrust detects this and sets go around thrust (in the all engines operating condition).

[Fat Dog]

timzsta

Yep, the A/T is armed during the before take off checks and therefore used to set TOGA thrust (T/O or R-T/O) for take off.
For climb, A/P vertical modes used are VNAV SPEED (note there is no path profile as such in climb, only FMC computed climb speed, hence VNAV SPEED), LVL CHG, or V/S (only normally used to control excessive V/S in busy airspace to avoid TCAS confilcts - 737's climb like lifts when light).
Note the A/T never controls pitch - only IAS in A/P V/S or N1 (climb thrust) in VNAV SPEED or LVL CHG.
On finals the A/T is not normally selected off - usually (operator specific) in either MCP SPEED or ARM.
Must admit I've never heard about the A/T detecting high N1 and selecting TOGA - certainly this could not happen if the A/T is off, it might be the case for A/T in ARM though.

Cheers
Fat Dog