737 Autothrottle: Disengage Vs Deselect

[galdian]

Hi all

NOT a question about the merits (or not!) of either - I've been de-selecting for 23 years - simply wondering in your 737 operation whether company policy (and strictly enforced or more a suggestion/guideline??) is:

- must disconnect;
- must deselect;
- either the above at PIC or Captains discretion.

With little explanation ie the following is gospel, trust me!

Mr Boeing states autothrottle disconnect as 737 is often the first jet for a company so KISS principle: all automatics ON or OFF, clearcut, no thinking required.
HOWEVER in an airline where the instructors and pilots are not all knuckledraggers Mr Boeing is more than happy for de-selection if adequately instructed and understood.

Just wondering if company policy is dictated by company size/type of operation/regional location etc or some other dictate(s)....however weird or wonderful!

Cheers

[Capt Claret]

Not a real Boeing but the B717 has autothrottle on from take-off roll to landing.

[A and C]

Deselect

i would deselect the Autothrottle when you start to fly the aircraft manually as if you have to GA you will get the Autothrottle to select the TOGA power giving you one less thing to do, this can be critical in avoiding a level bust if the GA level off altitude is low.

I'm sure that others can think of more reasons who deselecting is a god idea.

[Denti]

Company SOP is to deselect it. However it is the pilots discretion to switch it off instead which is usually the best option in gusty weather. Yes, it is nice to get TOGA power when you click the switches, but it is basic piloting to do it manually.

[B737900er]

You get low speed protection by deselecting.

[de facto]

this can be critical in avoiding a level bust if the GA level off altitude is low.

Really?please explain.

HOWEVER in an airline where the instructors and pilots are not all knuckledraggers Mr Boeing is more than happy for de-selection if adequately instructed and understood.

I agree,the mode must be understood.
One thing to bare in mind is as mentionned by Denti,in gusty conditions,the 1.3 vs may engage the AT into Mcp speed above VREF.
The mcp speed opens at wrong speed(higher) and pilot either has to switch off the AT or reset the speed and rearm it,the latter is tricky.

Also,the low speed protection is only working if the aircraft altitude is below the set MCP altitude.

[A37575]

Stick with the strength and the experts- the aircraft manufacturer. The FCTM states quite clearly "the autothrottle ARM mode is normally NOT recommended because its function can be confusing. Gust protection is not provided.

[Fullblast]

With A/T deselected, being very low on speed during landing wouldn't be the perfect stage for a tail strike if the throttles kicks out to keep the speed up??

FB

[STBYRUD]

The tailstrike issue in particular is why my company tells us to disengage the autothrottle entirely. To be fair though on an approach with a high chance of a go-around I tend to only deselect it until landing is assured to have a quick and easy way to set GA thrust if necessary...

[Capt Chambo]

5 operators over the last 20+ years.

3 disconnect
2 deselect

[c100driver]

There are two issues here.

Potential for tail strike with the auto throttle bringing in power at the flare with low speed.

Mode confusion. This is caused by the auto throttle going to MCP SPD mode. Some people think this is the speed protection that you get with de selecting SPD. If the MCP ALT is above aircraft altitude all well and good! However with the MCP ALT below the aircraft altitude the thrust will reduce. Just what you want when you are already slow.

Boeing is not "happy" with de-select SPD or they would not say it is not recommended.

To answer the original post, my airline is All Out or All In! We used to be De-select until we had a few issues on the line and then changed to Boeing.

[Denti]

Boeing as mentioned above does hand out FCOMS that advise to deselect instead of switching it off. If only for low speed protection and G/A thrust. However they only do that if they have audited the training program of the airline in question and are satisfied with the training program.

I always wonder about that tailstrike issue, it comes up every time we discuss about this issue here. However in around 25 years operating 737s with deselecting the autothrottle we never had a tailstrike on landing. If the autothrottle is active and the (left) RA indicates less than 27ft the autothrottle will go into RETARD mode from the current TLA. That is what helped those turkish airline guys to kill themselves and a few of their passengers in Amsterdam. The autothrottle will not try to keep the MCP speed in this special case.

When leveling off it will simply try to keep the MCP speed, if the pilot isn't able to do it himself in the first place. Something that sadly seems to be a major issue in todays world of magenta line wonder boys.

[de facto]

Presumably because once pitch mode enters ALT ACQ the A/T will reduce thrust to maintain MCP speed. Which in turn reduces pitch-up moment.

So will a trained pilot while approaching its altitude with the AT OFF.

Most pilots fly over the treschold at vref+5 rather than at vref.
Boeing guys may fly at vref from lets say 100ft to touchdown and having the AT in speed off mode may get to mcp speed more often than with line pilots flying a slightly more conservative vref+5.

In 8 years on the 737, i have never been nowhere near a tailstrike pitch due to AT engagement.
The only few times it quicked in was due to quite bad weather associated gusts and turbulence.
If the AT quicks in at lets say 50 ft,click click and go on...thats why one has a hand on the thrust lever on boeing aircraft..

[A37575]

I tend to only deselect it until landing is assured to have a quick and easy way to set GA thrust if necessary...

Another quick and easy way to commence a go-around is to simply push both thrust levers to go-around.... works just as well in a 737 as it does in a Tiger Moth

[A and C]

A37575

You are correct in theory but full go around thrust on the B738 gives a very large pitch up and is an excessive amount of power at light weights.

The chances are that if you push the throttles to the stops while selecting flap 15 and then get select the gear up you will have built up so much inertia in the aircraft that you will fly past a 1500 ft missed approach altitude, one click on the TOGA button gives a reduced GA power that makes the whole thing more manageable, two clicks will give you full GA thrust if you should want it.

[Fullblast]

Very true, but true as well that the PM should fix it, once the 15 flaps are set, the ballflag is around 90% for the NG. Again, same old story, you can do whatever you want if you know what are you doing...and why.

FB

[c100driver]

Boeing as mentioned above does hand out FCOMS that advise to deselect instead of switching it off. If only for low speed protection and G/A thrust. However they only do that if they have audited the training program of the airline in question and are satisfied with the training program.

All airlines I have had dealings with do not use Boeing FCOM but customise the generic Boeing FCOM to suite their operations so I would doubt that an unmodified factory FCOM will mention deselect.

If your airlines name is on the top of the FCOM then it is not a Boeing FCOM. However I have never seen a customised FCTM so that is really the unadulterated Boeing view of the world.

[uchy]

sorry for my ignorance, how you delesect the a/t?

[A37575]

The chances are that if you push the throttles to the stops while selecting flap 15 and then get select the gear up you will have built up so much inertia in the aircraft that you will fly past a 1500 ft missed approach altitude, one click on the TOGA button gives a reduced GA power that makes the whole thing more manageable, two clicks will give you full GA thrust if you should want it.

That is true if you are doing a coupled approach to touchdown with AT engaged. But probably the vast majority of 737 landings are conducted with a manual landing with the AP and AT switched off. That being so, there is no point in hitting TOGA on the GA since you still have to manually push open the throttles to climb away and all TOGA does then is give you FD GA bars which you don't need anyway unless you are incompetent at flying without the crutch of a FD.

[de facto]

That being so, there is no point in hitting TOGA

How about getting rid of those damn switches then?
Pressing TOGA is integral part of a GA procedure and to be followed.

FD GA bars which you don't need anyway unless you are incompetent at flying without the crutch of a FD

So useless that boeing makes sure those FD pop up during GA even if they were selected OFF...
Im sure you would enjoy flying a NO FD PRNAV GA...

A GA is a maneuver where one needs all available means to execute it properly.
If the FDs are available,use them,press those switches.
You can play the manual flying god another day...