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Auto throttle ARM Mode 737CL-NG

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Auto throttle ARM Mode 737CL-NG

Old 11th May 2010, 11:00
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Auto throttle ARM Mode 737CL-NG

Does your company use the Arm mode with the A/P disengaged?and if so, do your SOPS mention when to disengage it?(automatic at touch down or disconnect before the flare).

I see loads of good reasons to use it but just a few bad ones such as more chance to get a tail strike if strong gusts and not to be used during NNCs situations.

I heard BA uses the ARM mode, is it true?

DOES YOUR 737 AIRLINE USE IT?please mention your company name.

Thank you!

Posted for pure personal interest
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Old 11th May 2010, 12:49
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We disconnect auto-throttle when we disconnect autopilot.

Ryanair - 738
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Old 11th May 2010, 14:18
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Up till now my company SOPs dictate the A/T to be armed at all times, but now there will be change to the official Boeing procedures - including A/T disengage for manual landings. My two cents: I am a bit distrustful, A/T go-around is a very good thing, anything to reduce workload at low altitude is good - setting go-around thrust manually is a pain, and to my knowledge the 'tailstrike due to autothrottle intervention' is purely anecdotal, can someone shed some light on this?
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Old 11th May 2010, 16:31
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A/T tailstrike

You are right, its anecdotal what many think is that at flare if IAS drops way below the App spd then the A/T could kick in, MCP SPD and an abrupt uncommanded increase in thrust result in a pitch up causing a tail strike.

But no data or else Boeing should warn us about the same, advantages are min spd reversion and alpha floor protection. So I find it an excellent tool though let the youngsters kick off both in order to get hands on experience.
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Old 11th May 2010, 18:32
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If in ARM mode the AT will not try to hold MCP speed during flare. Below 27 ft it will rather go into RETARD mode. We do operate in ARM mode for at least 20 years, PM me for company if you really need to know.
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Old 11th May 2010, 18:38
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Good to see that I'm not the only one who likes the A/T on I prefer to deselect the speed (and request to have it deselected by the PM again in case of a level-off) during manual flight to losing all protections and automatic go-around thrust in order to prevent something that evidently hasn't ever occured anyway.
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Old 12th May 2010, 04:57
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FWIW

At a flight testing symposium held by Boeing for my airline many years ago this question was asked of the experts. Their answer was that the FCTM is quite correct and they will demonstrate why they do not recommend the A/T in ARM mode in the simulator. I was not high enough up the food chain to get to spend some time in the sim with them, needless to say not long after that my airline changed to All On or All Off for the B737.

The latest FCTM still has the statement "Autothrottle use is recommended during takeoff and climb in either automatic or manual flight. During all other phases of flight, autothrottle use is recommended only when the autopilot is engaged in CMD."
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Old 12th May 2010, 07:50
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I can verify C100drivers comments. I have asked the most senior people on the 737 program and they are as per the FCTM quote, ie A/T off for landing when the A/P is disconnected.

The reason is that unexpected/unnoticed A/T activity may destabilise the landing leading to a tailstrike or worse. If your airline still permits the use of A/T at arm and has an incident you may be on thin ice legally.

S&L
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Old 12th May 2010, 08:07
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Manual Flight = Manual Thrust for our Airline.
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Old 12th May 2010, 09:31
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FCTM also states that "the A/T ARM mode is NORMALLY not recommended because its function is confusing"
Obviously using the ARM mode,one will need to apply approach speed correction for wind.
But having used the ARM mode for the last 6 years without any problem(only profits such as automatic G/A thrust and having in the back of your head that you have alpha floor protection),changing this now feels a tad strange.
Using the ARM mode allows you to control the speed with a back up and never did i have the problem of having it going in the retard mode,anyways you still control the thrust manually...if your speed ever goes into MCP Speed,it means you are not doing your job and is a good wake up call,always better than going below Vref.

This arm mode is i believe useful for the approach phase when a G/A is possible rather than during a visual when i disconnect the A/T but ...rearm it on final for possible G/A
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Old 12th May 2010, 12:03
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Several threads on PPRuNe about this.

To precis (and as above)

Manufacturers do not 'recommend' therefore you may be on 'sticky' ground.

I have 3 times seen a high flare manoeuvre and A/T 'kick in' - it can, Denti. It can be hilarious to watch. I watched a fleet manager try to perform a low fly-by. Try it in the sim sometime.

In its favour is the g/a (plus possible avoidance of a THY at AMS if all is working). Without A/T on the g/a the nitpickers once told me you must apply TOGA with manual throttle rather than using a reduced power as per a single press reduced setting.
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Old 12th May 2010, 12:28
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Dear BOAC,

First of all, I would like to say i quite enjoy reading your posts.
Could you please elaborate on your last sentence concerning the need to use full TOGA thrust because of A/T off?
Nipickers...thats funny.I ll remember it
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Old 12th May 2010, 17:59
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Flattery will get you everywhere.

I was 'picked up' for doing a reduced power manual g/a with a stop altitude of 2000' in a very light 737 sim - should have used full power. When I had finished spluttering and cursing I was told it was a CAA requirement IRT-wise for a manual g/a - and there was no discretion. No doubt that was correct, but obviously I decided to say 'what a good idea - would you sign my licence please?' - and he did.
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Old 12th May 2010, 19:11
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(only profits such as automatic G/A thrust and having in the back of your head that you have alpha floor protection)
The B737 does not have an ALPHA floor protection, it only has min speed protection.

They are very different things and should never be confused.
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Old 13th May 2010, 04:46
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quote :'The B737 does not have an ALPHA floor protection, it only has min speed protection.'

C100 driver,


I am aware the 737 A/T in ARM mode gives minimum speed protection.

If you read your BOEING Systems Manual in the Automatic Flight chapter,under Autothrottle system/Autothrottle engagement states:

'Moving the A/T Arm switch to ARM,arms the A/T(.....)A general summary of A/T mode engagement is as follows':

6 points are given the last one being:'ALPHA FLOOR AUTOMATICALLY ENGAGES THE A/T when armed' which i understand being the min speed protection.Such as A/t will kick in if speed drops to vref

If i am missing some please explain.
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Old 13th May 2010, 09:02
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Yes I can explain.

Your version of the "Boeing Manual" is an airline customised version so someone other than Boeing has added an extra dot point. The only true document is the one that is on My Boeing Fleet, and I don't have access to that at home.

There are only five dot points in my Boeing Manual.

ALPHA floor is a term from Airbus (from their aircraft) which is the ultimate stall protection. I do not know enough about the Airbus to explain all the ins and out of its workings.

For the B737 the auto-throttle will increase thrust to maintain its minimum speed (the books say approx 1.3 v/s) however what the book does not say is that if the MCP altitude is above current altitude that is true however if below the current altitude then the thrust will close and the elevator will pitch down to minimum speed. Min Speed reversion is an MCP SPD mode.


From My company Boeing manual.

Autothrottle Engagement

Moving the A/T arm switch to ARM arms the A/T for engagement in the N1, MCP SPD or FMC SPD mode. The A/T arm switch is magnetically held at ARM and releases to OFF when the A/T becomes disengaged.


A general summary of A/T mode engagement is as follows:
A/T SPD or N1 modes automatically engage when AFDS command pitch
modes become engaged
Engaging LVL CHG or VNAV climb modes automatically engages the
A/T N1 mode
Engaging LVL CHG or VNAV descent modes automatically engages the
A/T in RETARD and then ARM when thrust is at idle
If not in a VNAV mode, engagement of ALT ACQ or ALT HOLD
automatically engages the A/T in the MCP SPD mode; otherwise the A/T
remains in FMC SPD.
Engagement of G/S capture automatically engages the A/T in the MCP
SPD mode.
Minimum Speed Reversion


The AFDS and A/T do not control to a speed which is less than minimum speed for the current flap configuration. This speed is approximately 1.3 Vs. Minimum speed, FMC speed, or selected speed, whichever is higher, becomes the AFS commanded speed. If actual speed becomes equal to or slightly less than the minimum speed, the underspeed limiting symbol appears in the MCP IAS/Mach display, and if operating in the V/S mode, the AFDS reverts to LVL CHG. The AFDS will also revert to LVL CHG from VNAV PTH, except when capturing or flying a level segment.
The AFDS commands a speed 5 knots greater than minimum speed. Selecting a speed 15 knots greater than minimum speed reactivates normal MCP speed selection control. The AFDS commands nose down pitch to increase airspeed if the thrust levers are not advanced. When actual speed becomes 15 knots greater than minimum speed, the underspeed limiting symbol disappears.
The A/P disengages, and the F/D command bars retract when in a LVL CHG climb with a command speed equal to minimum speed and a minimum rate of climb cannot be maintained without decelerating.
Minimum speed reversion is not available when the A/T is OFF and the AFDS is in ALT HOLD, ALT ACQ, or after G/S capture. Minimum speed reversion is also not available when in VNAV PTH and capturing or flying a level segment.

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Old 13th May 2010, 09:06
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Such as A/t will kick in if speed drops to vref
- not Vref, but some function of Vs. The system is incidence driven and therein, I believe, lies the years of confusion over 'alpha floor' which is as I understand it is an AB 'copyright' term.

As far as I have been able to ascertain it works on 1.3Vs, but I have not found more than informed opinion to support this. Whatever it is or is called, it is designed to protect against excessive AoA.

It may be of interest to study Vref, particularly at F40, where you will find 1.3Vs is remarkably close to 1.3Vs at some weights. I used to disconnect A/T when flying my (usually precise) 'Vref over the threshold' at F40 to avoid interference.
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Old 13th May 2010, 09:39
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Hi BOAC,

'alpha floor' which is as I understand it is an AB 'copyright' term
Not so. We had this protection on Lockheed TriStars in the 70s - PFM. With one hand on the TLs, we could feel when this protection came in - and manually dampen out any over enthusiastic TL movement. Can't you do this on 737s?

I don't have the manuals any more - but I think the threshold was 1.3 VS in normal flight and 1.25 VS on GA. Maybe 411A could remind me?

Last edited by rudderrudderrat; 13th May 2010 at 11:17. Reason: tidied up
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Old 13th May 2010, 10:36
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Another angle to this is that you'd like to fly pitch & power in the approch.
Pitch to keep the airpseed and power to keep the glide.
So a low speed problem is originating from something else than only a lack of thrust. Then to only apply a bunch of thrust as the A/T would do, would maybe bump your speed up, but will let you end up in a whole set of other troubles. I could easily see a deep landing result from this. The only correct act to save yourself from an unstabilzed situation like this is to go around and try again.
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Old 13th May 2010, 11:23
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Not so. We had this protection on Lockheed TriStars
- so you are saying it was called 'alpha floor' are you? That was the point of that part of my post, not that you 'had it'. If you have documentation to show that AB actually 'stole' the expression 'alpha floor' from Lockheed, so be it. Not really an important point, but one which confuses.
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