B 737 Autothrottle

[aviatorpk]

Do you disengage it on visual app. or deselect speed mode so that it is in armed state? can the B737 drivers give their opinion

[Slasher]

For 737-EFIS deselect SPEED mode and leave it in the ARM state after landing flaps are set and the ASI is where you want it.
Turning off the AT at any time during flight is ok as long as you know what protections you lose.

[2 and 70]

Flying manually with it engaged is OK - but not recommended due to pitch/power couple..

It's best to use it in "arm" mode when flying manually - ie: autothrottle on with no mode selected (eg: lvl chng, v/s etc.) On a manual approach, it's best to have it armed for alpha floor protection, and also TOGA available, just in case.

But... just disarming it (ie de-selecting speed mode) also ditches the flight directors - so if you are flying it, the other pilot can't monitor as easily by reference to the flight director bars - so you need to turn the autothrottle off, with the modes selected as normal so that you can get FD indications.

Make sense? Probably not! :)

[Sink Rate]

The trouble with leaving it arme but speed deslelected is that at some weights in gusty conditions alpha protection will cut in and out just as you are trying to plonk it on the tarmac. Sounds good in theorey but in practice actually destabilises the approach for the sake of some transient gusts. I often disengage it completely during the last 3-400 feet.

[Slasher]

2and70. Since when do the flt directors disappear when SPEED mode is deselected (putting AT in the ARM state)?

Could you give me an example where this would happen?

[Portly]

Slasher, you're right, and sink rate is too.

The fact that alpha protection is lost when in manual thrust is another disgraceful piece of design by the Seattle 'experts', but is probably not a daft as the lack of automatic stowing of the speedbrake on advancing the thrust levers.

Both features should be available, and an inhibit below, say, 100ft RA covers the point about alpha protection on landing. Of course, the 737 doesn't have this.

[2 and 70]

I may be wrong.... but.....

Say you elect to fly manually - no A/T, no FD on your side. So you turn your FD off, and deselect speed mode (ie A/T armed). This makes the other pilots FD master, and there is no FD pitch bar, since there is no pitch (speed) mode selected - since you deselected it to keep the A/T armed.

So to keep the FD on for the other pilot, a speed/pitch mode has to be selected and therefore the A/T has to be off to keep manual thrust. But this will only be the case when using a "path" pitch mode (ie: App, V/S, VNAV, Alt Hold)

Is this correct? Or am I mistaken?

I'm trying hard to put an example into words - but I tried it a few days ago, and to keep the other FD doing it's thing as well as me being able to have manual thrust - I had to have the A/T off rather than armed to do so. I'll see if I can come up with an example!!

[This message has been edited by 2 and 70 (edited 06 September 2000).]

[ManaAdaSystem]

2 and 70

You are mixing autothrottle modes with pitch modes. Autothrottle has a mode window but no flightdirector indications. The only flightdirector associated with speed only is the Lvl chg mode, where the throttles during desent are at idle and speed is maintained by changes in aircraft pitch.
You can manually fly the 73 using flightdirectors, with autothrottle engaged, deselected or switched off completely. You may fly with flightdirectors on either side or both.

[This message has been edited by ManaAdaSystem (edited 06 September 2000).]

[2 and 70]

I think I've found the correct example...

Flying a descent manually with your FD off.

You take manual control whilst in "Lvl. Chg" - disconnect the A/P and deselect the current speed mode, which is "Lvl. Chg" to give you manual thrust. But deselecting "Lvl. Chg" dumps the other guy's FD pitch bar, so if you reselect "Lvl. Chg." he get's the pitch bar back and you (for the moment) still have manual thrust (A/T annunciated as "Arm"). But when the aircraft goes from "Lvl. Chg." to "Alt. Acquire" / "Alt. Hold", the A/T will advance the thrust levers - which is not what you want. So to fly with manual thrust in the descent, you have to have the A/T off to keep the other guy's pitch bars and to give you manual thrust.

Have I got the right end of the stick here?

[ManaAdaSystem]

If you are flying manually during desend in LVL CHG mode, deselecting autothrottle will do just that. You will still have the flightdirector pitch bar present, it will command the speed set in the speed window. To maintain that speed you must adjust aircraft pitch accordingly. You may turn the autothrottle off completely, and you will still have the pitch bar.

[2 and 70]

Not convinced - but I'll give it a go next time and see what it does! :) But I'd have though that no pitch or speed mode selected (ie Lvl Chg deselected) there will be no pitch bar.

Thanks for the replies.

2 and 70

[ManaAdaSystem]

You are absolutely correct. If you deselect LVL CHG you FD pitch bar will disappear, because you are deselecting your pitch mode, NOT your speed (autothrottle) mode. Ie, you are pushing the LVL CHG button, not the SPEED button.
LVL CHG is a bad example, as in this mode the autothrottle will RETARD and ARM itself automatically. Imagine a manual FD ILS, deselecting speed now will deselect the autothrottle only, it will not remove the pitch bar.

[This message has been edited by ManaAdaSystem (edited 06 September 2000).]

[This message has been edited by ManaAdaSystem (edited 06 September 2000).]

[2 and 70]

All cleared up then!

So... How do you manually fly a descent with the A/T armed?

From what we've sorted out here, you can't (assuming you're using Lvl Chg for the other pilot's FD) - or can you?

So is the only way to turn the A/T off for the descent until approach, and then Arm it for the approach to provide A Floor, TOGA etc. ??

http://www.pprune.org/ubb/NonCGI/confused.gif ????!!!

[ManaAdaSystem]

Of course you can, if already in descent LVL CHG and trottles in idle (ARM), just disconnect autopilot and follow FD pitch bar. If in V/S you must disconnect autopilot, deselect SPEED and follow pitch bar. In both cases you can (must) manually operate the trottles as you wish and you can turn FD`s off on either side (FD bars will then disappear on that side).
As long as the thottles are armed you have alfa protection and they will engage if you press TOGA.

[aviatorpk]

I like to keep the A/T speed mode disengage so that in case of goround I have the A/T by the way F/D has nothing to do with A/T.

[Slasher]

2and70. I dont mean to sound rude but are you a 737-rated pilot? If you are its time you got back into the books and have a sim session mate! Your asking a lot of basic questions that you should already know, are very unsure of yourself and a few of your statements are pretty inaccurate.

As any properly endorsed 737-EFIS pilot will tell you, the only time both FD bars will disappear is when in a LVL CHG climb with a commanded speed equal to minimum speed and a minimum rate of climb cannot be maintained without decelerating. The AT will disengage itself too.

The other times all the FD bars will automaticaly disappear is when:
* ILS signals become invalid
* FLARE annunciates (green)

[2 and 70]

Ouch! I thought this forum was here to discuss items like that??

I'm by no means and experienced 737 driver - but I am asking a minor question that I can't find the answer to from my flying or tech manuals!

All I want to know is - Can you fly a manual descent, with manual thrust, with no FD, with the A/T armed and the other FD showing pitch to maintain the MCP speed (ie Lvl Chg) or Alt Hold.

From what I have looked through, the answer would appear to be no - you have to have the A/T off, to prevent thrust levers advancing at "Alt Acquire" from a "Lvl. Chg" descent.

Come on - shoot me down in flames! http://www.pprune.org/ubb/NonCGI/tongue.gif

[Portly]

2 and 70, you have my sympathies - you are being let down by a very poorly designed aircraft system (the autothrottle system), and also by a very poor set of manuals (from the aircraft manufacturer).

ManaAda-something is wrong to suggest flying a manual descent with A/T in ARM, for the reasons given above. You must not do this, because the automatic application of thrust with ALT ACQ will destabilise the hand-flown level-off.

Most training pilots will advocate 'keep it simple, stupid', and so I offer my advice to all of us unfortunate enough to fly this shoddily-designed aircraft.

If you wish to fly manually, with FD's on either or both sides ON or OFF, then you should de-select the Autothrottle. Do not de-select speed mode only, as this gives rise to the problems mentioned above.

Alpha protection is there do to a very specific job, and the Boeing version of the system is very much simpler than certain well-designed manufacturer's equivalents.

Fact: people's lives have been saved by alpha protection (two friends of mine amongst them).

Fact: if you feel you should have the alpha protection available at all times in a 737, then you should never fly the aircraft manually. There just isn't a satisfactory way of getting around this.

[This message has been edited by Portly (edited 07 September 2000).]

[cptn-bat]

The original question was if we deselect speed mode of autothrotlle to keep it armed but control thrust manually during short final i guess and not at top of descent.It is nice to have it ready for toga but some airplanes in our fleet engage just before touchdown in retard mode and it might slam you down if you not notice in time

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trust your cptn but fasten your seatbelt

[PPRuNe Towers]

I find all Portly's comments regarding the 737 devastatingly accurate.

While I'm sure there will be some itching to defend this aircraft's systems philosophy, ergonomics and manuals I'd love to know what other western designed jet transports you're comparing it with.

Yes, yes, I know it makes a profit and there's lots of them.

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Regards from the Towers

[email protected]

[ManaAdaSystem]

Hi Por-something.
Im not suggesting anything, he asked if its possible, and it is.

[Portly]

ManaAdaSystem,

Sorry about the '-something' in my earlier posting - but having got to the point of writing my posting, I couldn't go back and check your username, which you have to admit is perhaps not the most instantly memorable! My best regards to you, anyhow.

I'm not certain whether the question from Pprune Towers was directed at me, but the aircraft with the better design are basically Airbus, though the very best flight deck I have ever used was on the Fokker 70/100, which had a design and operating philosophy which leaves everything I've seen before and since standing. Moreover, the flying controls were still mechanical with hydraulic power.

Incidentally, it was in a F100 that my friends lived thanks to the Alpha-floor. Scenario was a fast, high, approach with the FO flying. Configuration was Flap 42, Gear down, Speedbrake fully out, Approach Idle thrust.

Feeling uncomfortable, the Captain instructed the FO to go-around, but the FO reverted to previous type and de-selected autothrottle (switch on end of thrust lever grip) instead of TOGA (trigger under thrust lever grip). He then wondered why the FD hadn't commanded a pitch up, so pitched up to approximately 20 degrees. Fortunately, the Alpha floor activated immediately and the Speedbrake re-stowed automatically, meaning that they recovered by about 200'AGL about a mile out from the runway. I am aware that this scenario has occurred more than once. In a 737 they might not have been so lucky.

[PPRuNe Towers]

Question was rhetorical rather than directed at you portly. I genuinely agree with everything you wrote.



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Regards from the Towers

[email protected]

[ManaAdaSystem]

No offense taken Portly!

[Slasher]

Im not sure what all the kaffufle is about.

2and70 whenever I have a question the books cant appear to answer, I generaly answer myself by trying it out the next time I fly the aircraft. Your question: Can you fly a manual descent, with manual thrust, with no FD, with the A/T armed and the other FD showing pitch to maintain the MCP speed (ie Lvl Chg) or Alt Hold? I confirmed the rationale the books say, on descent into Ha Noi last night. I tried it. And YES, you can. QED.

About Portlys comments (I find them on the money but not "devestatingly accurate"!):

Doesnt it all come back to knowing your aircraft? Despite the 737s shortcomings (and I agree with you, there are many of them! See just a few Ive listed below), the Boeing philosophy is: if you take out the autos then its your aeroplane. (which is what I infered in the last line of my 1st post). This is exactly how it should be - take out the autos and they will not interfere. Who wants something cutting in when youve taken it out? Confusion would reign if they did, and such confusion could lead to an accident. There is inherently nothing dangerous with hand flying with FDs, AT etc all taken out, so long as you realise what you are doing and whats going on esp with regards to the auto-protect function of the AT.

And mate I hand fly sometimes on descent with the AT in ARM, and dont have problems with the ARM to MCP SPEED transition at ALT ACQ. Wheres the "destability" you speak of (unless your asleep)? I just anticipate thrust with pitch and keep a safe hand on the throttles as a backup.

I very often fly an entire sector raw data with no FMC or FD or AT from push-back to taxy-in with absolutely no problem. The only brief I give the FO is to double-crosscheck all MCP selections, make every standard callout AND call them clearly (not just a grunt or mumble), and be aware of loss of AT speed-protect.

And Portly I realise I wasnt there on the Fokker event you discribe so there might have been extenuating circumstances, but isnt it a universal policy that someones hand is kept on any SB lever whenever its deployed? Im kept fast and high in the 737 almost everyday and in high workload situations (often in severe wx) and yes I agree the SB lever can be easily forgotten. But if some buggers hand is there its nipped in the bud before theres any trouble.

By all means take advantage of the protections offered in your aircraft if you want them. Thats what they are there for. There are times certainly when its good airmanship to utilise them to the full. But dont let your standard sink to the point where you solely rely on them.

PS With regards to 737 shoddy design Ports, Ill certainley hand you that bloodey rudder, as well as AP, the inbuilt Go-Around logic (2 eng), lack of SB autorestow at min protect speed, APU, autobrake RTO logic, the lack of reliability of the AT ARM mode at ALT ACQ when hand flying (unpredictable because it sometimes just stays in ARM even through to ALT HOLD), the uselessness of the speedbrakes esp at speeds at 240 knots IAS or less, the lack of really important info in the books (QRH is a bloodey disgrace, and Vol 2 was written it seems with an absolute minimum need-to-know philosophy) to name just a few.

Back to bed.

[CaptainSquelch]

Slash,

It really is a lousy design and if you don't keep an eye on her she'll bite you in the @rse. But who's perfect. I just love the b!tch.

Sq

[Buzzoff]

Slasher, agreed about the hands on the SB lever, but the F70/100 one is a tiny little affair, more like a switch, unlike the 737's handle - it's not as easy to rest a hand on.