B737 Classic- A/T de-select speed
 

After a long break, I'm currently re-qualifying on the B737-3/400.

Back-in-the-day with another operator, we had an often used procedure of A/P disconnect, but rather than fully disconnecting the A/T, we would de-select speed, leaving the A/T in Armed mode, the theory being it gave low-speed protection.

Later I became aware that Boeing recommended against this procedure, as they thought it risked un-commanded A/T movement at critical times(such as flare/landing).

Lo and behold, my new company also uses this procedure.

Does anyone have a link to an actual Boeing guidance source recommending against this? I'd just like to diplomatically show my management that the manufacturer doesn't like this procedure.

I would check your training manuals, Wizofoz for the reference which states that Boeing recommends manual flight with A/T during takeoff and climb out only. There have been more than a few SBs on the A/T system for the Classic as well.

I have been told that aircraft energy management is the reason behind the recommendation. The A/T system is designed to work in concert with the CMD functions of the A/P. Manual pitch changes can lead to an undesirable (high) energy state during landing when A/T is engaged, particularly during gusting conditions.

You can also end up needle chasing (airspeed) when you get out of sync with the A/T control loop and a sort of hysteresis can occur, a condition that is extremely rare with the A/P engaged.

Thanks VA- But not sure we are on the same page. De-selecting the A/T puts the system into ARM mode, meaning it does not control the thrust UNLESS it reverts to SPD due to "ALPHA FLOOR" (Which is really min-speed)

I do have the reference in the FCTM which seems to not recommend using ARM mode, and will raise this with management- just wondered if anyone else had a Boeing reference that discouraged the practice.

Not a Boeing reference but our manuals (which I can't link to sadly) corroborate yours.

Still, it's a practice widely used and not discouraged. Go figure.

"I see", said the blind man. Apologies, Wiz. I think my eyes may have glazed over.

The change in A/T ARM mode guidance was fairly recent, like in the past decade or thereabouts. Wish I could be more specific, but that might be a good place to start looking. I am fairly sure avoiding mode confusion was one of the reasons behind the change.

AFAIK Boeing only recommended NOT deselecting as often 737 was first jet type for turboprop operators so KISS...all manual or all auto.

If an operator had proper training on the benefits (alpha floor and G/A thrust with TOGA) and the negatives/times should be disconnected completely (gusty/swirly wind conditions with large airspeed fluctuations) then Boeing lets you fill your boots.

It IS only a recommendation, not a requirement and the FCTM only a reference guide - although a bloody good one and grossly under used IMHO - as opposed to company SOP's, whether stated or implied.

Personally additional protection (safety) for (say) 95% of the time Vs less protection (safety) for 100% of the time....know which I'd choose.

Cheers.

I was unable to locate that additional "advice" you quote (?) from the Boeing 737 FCTM about "as often 737 was first jet type for turboprop operators so KISS..all manual or all auto". Someone has been pulling your leg, methinks:ok:

The FCTM does state: The autothrottle ARM mode is normally not recommended because its function can be confusing. The primary feature the autothrottle ARM mode provides is minimum speed protection in the event the aircraft slows to minimum maneuver speed . Other features normally associated with the autothrottle, such as gust protection, are not provided. The autothrottle ARM mode should not be used with non-Normal Checklists. Some malfunction that affect maneuver speeds cause the autothrottle to maintain a speed above approach speed.

If your company chief pilot wants you to use the ARM mode for approach and landing, then he would be wise to contact the manufacturer and request their advice. In turn the manufacturer will likely say read your FCTM or maybe even "fill your boots" and quietly advise your insurer on the side:E

Well....all those decades ago as a new, humble F/O on the 737 I didn't think it was my position to tell my leaders they were talking :mad:

The explanation made sense - still does - and de-selecting appeared to work well in the airline and has worked fine for me for over 25 years.

Pleased to see the words "normally not" in your FCTM statement, makes one wonder when Boeing WOULD recommend...maybe if proper ground school/SIM/line ops training and demonstration was completed, sadly appears either modern FLT OPS departments are incapable of including such training or maybe it's just too technical a concept for the modern pilot to understand?

Now back in MY day....

Anyhow whatever the wisest of the wise in your FLY DEPT bang on about will have to suffice.
Cheers. :ok:

Fascinating. I was taught to have an occasional glance at my airspeed during approach. I must be getting old.

Can you put the autopilot into ARM mode so it will flare if you don't get around to it?

In all seriousness, I can think of a couple of recent 777 crashes caused by the pilot expecting the autothrottle to move and it didn't... Maybe mixed automation isn't all it's cracked up to be.

Is it mixed automation?

Maybe it's more back-up/redundancy which improves....safety.

In ARM mode - apart from excessive wind gusts when consider disconnecting as discussed - the A/T will only engage if the pilot :mad: up and allows speed to decrease towards the stall.
And that's only after it flashes a warning for a while to alert the pilot of what the A/T MIGHT have to do if the pilot doesn't correct THEIR error.

A system that tries to prevent the aircraft from stalling if the pilot gets distracted - mighty dangerous piece of equipment, better not use that!

At any stage - regardless of what systems are/aren't being used - the pilot has an obligation to make sure what should be happening IS happening, if not then MAKE it happen.
If pilots decide NOT to make things happen....well that's a whole other thing.

And some crooks have been breaking into houses for 25 years and not being caught. That doesn't mean to say it is legal and safe:E

Seriously, though..one sequence where selecting ARM on the A/T for manual approach and landing can bite you in the bum, is the All Flaps Up Landing.
This might be one of the sequences cautioned by Boeing when it says:
The autothrottle ARM mode is normally not recommended because its function can be confusing

Thankfully, few pilots have ever been faced with an All Flaps Up landing in the real aircraft in their career. Me for one. But in the simulator I have observed on many occasions where a manually flown all flaps up landing is conducted with ARM mode selected, the throttles started to open un-commanded seconds before the flare as speed is just on VREF. Of course a wise pilot will firmly hold the throttles to prevent this happening - but not all of us are wise and the creeping open of the throttles at the flare might be unexpected.

Which is probably why the FCTM adds: The autothrottle ARM mode should not be used with non-Normal Checklists. Some malfunction that affect maneuver speeds cause the autothrottle to maintain a speed above approach speed

More than fair, Centaurus!

If your flying around constantly referencing the NNC then A/T disconnected may indeed be the way to go.

HOWEVER....if you're flying around constantly referencing the NNC there are probably bigger questions you should be asking about the professional airline operation, the A/T question should pale into insignificance!

All good fun

I'm just not so sure that having it in ARM as "protection" is really as safe as you make out.

The pilot "knows" it's in ARM, so may be expecting it's protection. That could be more dangerous than it's worth. I consider it bordering on mixed automation, which has already killed two 777's.

Sorry, I'm a big fan of "all on" or "all off". At least you know where you stand if the situation suddenly goes pear-shaped.

Sorry, I'm a big fan of "all on" or "all off". At least you know where you stand if the situation suddenly goes pear-shaped.

That's the Boeing idea, and generally it works fine. There's always a but. I used to fly B733 in 90's and in an RA it was A/P off. No mention of A/T. I moved to B738 and it was all off. I then checked back with the up to date B733 book and it had changed too. When I asked a Boeing pilot, he repeated the Boeing mantra. I've seen too many low speed events with high level RA's as PF forgot to reconnect A/T and PM was still breathing fast. Low interest to those comments.
On finals I'm not a fan of anything that reduces PF's involvement with manually manoeuvring the a/c and dulls the loop of PF-aircraft hand- eye coordination. Derfred has a good point. Don't encourage PF to reduce their scan of what's going on. It is not a playstation and needs positive control of all parameters, and speed is one the more critical. The pilot's brain must be well ahead of the a/c and therefore sharp & alert.
How many operators make A/T mandatory for LVO approaches? Some do. It's not a Boeing thing, but if airlines begin not to trust PF's skill in maintaining speed in manual flight, they might carry that across to auto flight, unnecessarily technically.
There is also the case to consider of not allowing pilots to operate under conditions that may be allowed and expected under MEL guidance. A u/s A/T is not a NoGo, and it is a single system i.e. no back up. If crews are not used to it being off, but are then expected to operate a whole flight in manual thrust, it might prove a little too much for some. We've seen the consequences of mollycoddling pilots in too much automation. Considering my early days in B732 and last days in B757/767/738 I've seen a wonderful evolution of automatics and enjoyed many of the improvements on offer; and never felt I'd lost any control. What did make me feel emaciated was mollycoddling SOP's.

We all have stories where "things happen" with various systems engaged.

A go around in min wx by a "new" old esteemed Captain where he'd disconnected A/T, applied way too little power and ended up with speed decreasing with a SINK rate of 800fpm below 1000ft AGL before the (fortunately) experienced F/O was able to "assist" the Captain in recovering without having to take over and all the "face" issues that would have caused.

One wonders what would have happened with a newer, more socially correct F/O.

He pressed TOGA correctly, A/T in ARM would have been a non event instead of far too close to a smoking hole.

RAT 5 you have me confused - I think you're saying A/P off and A/T on which I totally agree is no go - but A/T in ARM means manual thrust/speed control by the PF up to the point where the PF has stuffed it up, lost attention/scan, speed decays....and the ARM mode kicks in to 1. advise the PF he's stuffed up then 2. prevent the aircraft from stalling.

ARM is not an active mode - unless the PF stuffs up and tries to stall the aircraft.
PF doesn't stuff up - the ARM mode has no need to do anything, will not interrupt manual thrust operation and will click itself off 2 seconds after touchdown.

Pure Boeing magic! :ok:

Galdian: apologies. I confused myself. Indeed, as you say. Deep in my memory, and playing hide & seek, is an item from B757/767 days where there was the ARM function in daily use. (I stand to be corrected). The reason I think I remember this is when I transferred companies onto B733 there were other B757 pilots who experimented with the 'hidden' ARM function and found it seemed to work. In that embryonic airline they discussed its use within the training dept only to be rebuffed as a no no. It was taken as a case of the people being spoken to were less experienced that those speaking and didn't understand the question. So the Ops guys went to Boeing and received the standard Boeing 'not recommended' and thus it became a DO NOT. On the outside it seemed the B757 AFDS was the same as B733, but we all know there are gremlins lurking behind the facia deep in the electronic bowls of the E&E bay. What seemed to be happening in Seattle was that the B737 fleet development dept operated separately to the very similar looking B757 dept. They introduced some similarities, perhaps in the hope of common type ratings, but not everything. (who ever thought that using the flap lever to drive the speed bug on GA was good idea. Duh!) I still think B737 ARM is a grey area. It might well work and work well, but I wonder if there are certification problems that are better left unopened? As a result, 'not recommended' is interpreted as 'not allowed' by the many, but not all. If some XAA's approve it what argument do those who deny it use?

Interesting points made Rat 5. It becomes a problem when type ratings are undertaken in the simulator and students are actively encouraged to thoroughly study the various FCOM and FCTM.
Depends too on the simulator instructor's previous background. So one instructor says its OK to de-select speed and leave the AT armed. Another says just do what the Boeing books say. There are no crashes either way so the student decides on the safety of contraception - i.e.de-selects speed just in case so he won't stall on final.

There are pilots who get twitchy without the crutch of some sort of automation. I have seen them flying visual circuits in the simulator with the flight director on, using split cues - that is pitch bar showing altitude hold and heading mode off. The fact that the altimeter is the primary pitch "bar" for level flight escapes them. Thus their addiction to the automatics need another aid - the FD pitch bar. I could be naïve, but IMHO Boeing have carefully considered the pros and cons via extensive research before committing recommended techniques to paper. That presumably includes switching off the autothrottle system when manually flying on final approach in a 737

Conflicting information during training is of course a big no no. I flew the 737 classic and NG in airlines that used the ARM function, consequently the FCOM was adapted as well as the training to reflect that, the boeing warning was not contained in the FCOM anymore and every trainer advised us to use the ARM mode, and we were trained in its behaviour. Worked like a charm and had some positive and negative sides. In weather with strong gusts we tended to switch the AT off completely as intermittend autothrust activation above bug speed happened.

We didn't see it as a crutch, but rather as a safety device, and funnily enough boeing did agree on its frequent audits.

After years on 737-100,200,300,400,500,800...
 

Just a personal perspective. I never saw any useful benefit of that use of A/T. I always preferred either an A/T ON or OFF control of the engines, also resting my hands on the thrust levers to tactfully confirm they both are doing what I want them to do. Back on some prehistoric 300s we saw that one thrust lever would come up whilst the other stayed at idle. A real surprize that prompted me to leave my hands on the thrust levers almost all the time I was expecting them to be moving either way. On another but slightly related subject, use of CWS was very useful at times, but Boeing now says NO to that and our 800s will not go into that mode via overpowering the control wheel or column.

stator vane

with A/T in ARM you have as you wish...manual thrust, you can push and pull to your hearts content, the A/T will have absolutely no input until the PF tries to stall the aircraft...like on a min wx circuit at night when it's all happening, get a bit distracted and the airspeed drops off,

And that's after it's done you the courtesy of saying "hey fella you're going to force me to do something unless you fix YOUR error".

ARM is NOT an active mode, it is a back up and a redundancy so, by definition, improves safety.
And as soon as I mention the word "safety" I win, end of story! ;)

Denti - nice explanation, thanks.

I think an important point to emphasise is that you should be trained and the use (and times when NOT used) demonstrated in the SIM and discussed at suitable times during line training, get the impression it's not being trained then the wisest of the wise in Flight OPs say "why are we having problems"??