Autopilot uncommanded disconnect - certification requirements.

[RMC]

Hope someone can help. I was flying my 737 NG in max rate mode (trying to meet an ATC restriction) ...was at least 6,000 feet below max cruise alt wings level and still above the amber band (start of min manoeuvre speed) when the autopilot disconnected and the aircraft pitched significantly nose down. What are the certification requirements of an autopilot in this regime....the engineers are just saying tested OK nothing wrong with it.

[300-600]

NFF - The line engineers favourite abbreviation. The fact is that in an NG below optimum you can actually climb in the amber band....just don't go doing 30 degree banked turns!!! The autopilot should not disconnect in the circumstances you describe.

[Intruder]

FAR 25.672 has the basic airworthiness certification requirements. They include an appropriate warning that must alert the pilots to failures.

AFAIK, in practice that extends to the autopilot disconnect alarm sounding EVERY time the autopilot is disconnected, intentionally or otherwise. That is why the second press of the disconnect button is required to silence the alarm. if the alarm did NOT sound in your case, I believe that would be a grounding discrepancy.

[RMC]

Thanks guys....anyone like to comment on whether you would expect the autopilot to be disconnecting 10 knots above manoeuvring speed (top of amber band in the NG case).

[kijangnim]

Greetings,

737 and 727 200 , if , using a spoon you hit (not hard of course) the control colum the auto pilot would trip, this was more than 20 years ago and we still dont know why :confused:
Aircraft pitch down and autopilot trip could be a transcient from ADRU altitude increase to which the autopilot will respond to a pitch down to maintain the demand, and then disconnect, investigation will; as one colleague wrote close the matter with NFF.
Auto pilots are certified using DO 178B level A, this Design Order is highly constraining for hardware and software, specifically when it comes to CAT III.
Level A means NO FAILURES hardware, or software within MTBUR, to the exception of fail passive and fail operational for CAT II/III B.
The component of the board processing the signal are different from the board monitoring, the power supply is different, and the software is different on the boards, and all boards are physically segregated from one another:ok:

[Checkboard]

Autopilot doesn't disconnect. Just above 1.3 Vs for the current config, it enters Command Speed Limiting mode, shows the underspeed symbol in the speed window (A for Alpha - symbol for angle of attack) and uses either pitch or thrust (whichever was engaged) to maintain that speed (as best it can). A speed 15 knots greater than than the limiting speed needs to be selected to remove the symbol.

[RMC]

Kijan pointed out that there was a history of AP disconnects for something other than what it says in the book...if I hadn't known that it wasn't one of the 12 things an autopilot should disconnect for then I wouldn't have snagged it!

[Checkboard]

I read the question. You asked:
.. would expect the autopilot to be disconnecting 10 knots above manoeuvring speed ..
I answered.

[BOAC]

Just a wild guess - it may have reached max elevator to maintain the attitude for the speed at that height? If it then had insufficient nose-up available....? Was it a steady speed climb or decaying?

[RMC]

BOAC...it was in max rate climb but was essentially maintaining mach 0.70 for the minute or so prior to disconnect. What do you think?

[411A]

it may have reached max elevator to maintain the attitude for the speed at that height

This can be a problem with older autopilot types, even the NG aircraft have a rather basic autopilot, carried essentially over from the earlier models, with minor improvements...at least according to a good friend of mine who was an instructor on the NG series.
In fact, I asked him specifically about this.
Also, he mentioned that there was just one aircraft that he had flown (and is now back flying) which had the very best autoflight system he had come across, one that does not trip either off nor to CWS in the event that the elevator runs out of authority (all-flying stab, elevator mechanically connected thereto), so these aforementioned nuisance trip problems simply do not occur....out of trim annunciated for the pilots, yes, well in advance.
The design is over thirty five years old, and still works faithfully.

TriStar.

[411A]

Is there no thread you won't hijack to drone on about...

Quite likely not.;)

If they're that wonderful, how come they only operate in the third world?


Actually, ASFKAP, several still operate from Portugal, and the last time I has there, it was still a member of the EU.
Several still in the UK, as well...RAF.

[BOAC]

Dragging this back on topic from the 'Aviation History and Nostalgia' gang, I don't know, RMC. Either you had a duff A/P or it reached some limit and appears to have been out-of-trim (nosedown) at disconnect? We need some avionics input here from an NG qual techie.

[john_tullamarine]

.. come on, guys .. if you don't cut 411A some nostalgia slack .. I'll start reminiscing about the halcyon Electra days (another fine Lockheed product) ... and probably even get a tad maudlin that I didn't get an opportunity to fly (although I had a few pax rides in) the larger Lockheed product ...

So, what's it to be ... one bloke reminiscing (and invariably politely) ? ... or two ?

so great that the crews would be still talking about it ten years after they'd been withdrawn from service....

touché, good sir .... brilliant ...

[el #]

Even the beloved machine got its quirks sometime:

http://www.tristar500.net/articles/cstmp_stansted.pdf

Did the pilot in LH got command ranks after all ?

[411A]

Even the beloved machine got its quirks sometime:



Yes, it does, however, if flown properly, these problems don't develop.
Case in point.
This nonsense of having the First Officer fly the approach, then hand over for landing to the Captain, can provide difficulties, and I expect this was the outcome, as a First Officer (upgrade to Captain) occupied the LHS.
Also, once no dual is annunciated at a low altitude, the affected tripped off autopilot (if it does trip off) should not be re-engaged, as otherwise problems can occur, especially if autothrust is engaged at the same time.

When in doubt, follow the Lockheed supplied Aircraft Flight Manual, which clearly indicates the way the airplane should be operated for automatic approach/land maneuvers.

RTF (Lockheed)B...for best results, as otherwise pilots are just asking for trouble.

And, this does not just apply to one type either.
The aircraft manufacturer goes to rather great expense to indicate how the airplane is to be flown and the procedures to be used...problems arise when airlines 'think' they know better.:rolleyes::rolleyes:

[bflyer]

Hi guys

I must say that the civilized fencing going on here is providing us upstarts with some great technical and operational knowledege in the process

ASFKAP...411A....please do go on:ok:

[john_tullamarine]

.. one of the inevitable consequences of being olde pharts .. is that we pick up these and those bits of information and knowledge along the way ...... a strength of PPRuNe I might suggest ...

[BOAC]

So, what's it to be ... one bloke reminiscing (and invariably politely) ? ... or two ? - no objections, John, on the proper thread? While you are cutting posts, could you delete RMC's double post since he seems unable/unwilling/(unaware?) to do so?

May I respectfully suggest, as an ex-mod (and an olde phart), a thread on how wonderful the 1011 was (correctly in AH&N, of course), and this one on A/P disconnect in the climb, since we do not yet appear to have an answer apart from ASF's?

[john_tullamarine]

BOAC,

(a) thanks for the observation re double post .. I'd missed it altogether .. now merged and tidied up

(b) suggest you shoot a suggestion to 411A re a nostalgia thread on the Type .. he'd have to be the best man to start such a thread ... I think that we'd have Buckley's of getting him not to put the occasion L1011 plug into threads ....

[BOAC]

I agree with your assessment. We have all learnt to ignore those bits, however, have we not? It is the thread diversion which follows to which I directed my request. You only have to see what a shambles R&N can become.

[WX Man]

Just out of interest, is this a requirement of FAR 23 as well?

[hawk37]

BOAC, could it really have reached max elevator? Surely in that case the AP would normally have trimmed the stab, to relieve the elevator force. Or is there a flight envelope for the 73 NG with AP on that is different when hand flying?

RMC, on most aircraft the AP will disconnect with a certain force on the yoke/control assembly, releasing the controls in their out of trim condition. Sounds sort of what you had, although your's may not have been as extreme. On modern aircraft though, the AFCS normally sets off a mistrim message before the force gets to the point that the AP will let go. This alerts the pilot that all is not well. Does the 73 NG have such a system?

[BOAC]

Hawk - I have no idea why it happened, but was just throwing in a 'query' since we appear to have a serviceable A/P misbehaving.

RMC - which mode were you in? LVLCHG or V/S?

[NinerVictor]

RMC mentioned that he was maintaining M0.7 in a climb. Is it possible that the IAS dropped below the minimum drag speed? As the IAS continued decreasing with M0.7 being maintained in a climb, there is more drag (back side of the drag curve). So the aircraft becomes thrust limited and cannot maintain the climb, hence the A/P tripped. But RMC also mentioned that he was at least 6000' below Max Alt? I'm don't know much about the 737 FMC, so I'm just guessing here. Was the vertical speed decaying through the climb?

[RMC]

Thanks for all the feedback so far.

BOAC - Level change.

N V - min drag speed 0.73 ...the drag at 0.7 is less than that at LRC...drag starts increasing significantly around 0.66

[el #]

ASFKAP, you ruined everything. My dream was to become super rich and have an airline exclusively made of trident's just for fun.

They would have practiced automated cat IIIb landings with 30 kts of x-wind all the time and made fun of they guys that had to divert.

I also imagined I would have hired 411A as operation's chief and have a bottle with him twice a week while listening to his old aviation stories. His time would run out once my hostie-dressed playmate driver would came to pick me up for going to bed :)

You really broke my dearest dreams and I don't know if I will be ever able to imagine something nice as the above.

[411A]

Perhaps you could have a 'sticky' in each forum where 411A can post all his little anecdotes about being an L1011 captain and how wonderful the now defunct airplane once was etc...

Now defunct?
Hardly, for some charter operators who continue to make a handsome profit with the superb product.

I worked on it for years, the issue of how wonderful an airplane it is is entirely subjective, we thought it was an overcomplicated under reliable piece of crap (evidently we weren't the only ones), the tail may have been a work of art but the wings were crap, the toilet design was crap, the hydraulics were crap, the electrics were just plain silly, even the IFE was total rubbish.

Quite strange.
Delta was the largest operator (69 L1011's at one time, as I recall) and a senior VP there told me personally, and I quote...
"The L1011 made more money for Delta than any other airplane in our fleet, at the time"
FACT.
So, it would seem that those who had only a few, never really learned how to maintain the L10 in the first place.
Why am I not surprised?:E

Sorry for the interruption, folks, back to the question at hand.:}

[john_tullamarine]

Folks ..

(a) let's not have the thread turn into a vendetta .. lest it be locked

(b) it would be a problem if we were not able to tolerate a small amount of nostalgia or any other form of eccentricity in the forum

(c) for those who don't wish to read anything of a particular flavour .. it is a very simple exercise to skip over the material concerned ....

[411A]

(b) it would be a problem if we were not able to tolerate a small amount of nostalgia or any other form of eccentricity in the forum


Indeed so, JT, so I shall keep is short.
I have generally found that those who are unable to understand a particular bit of machinery, constantly throw stones at it, in order to cover up their own inabilities to master the technology involved.

IE: generally called, sour grapes.;)

[411A]

Back to the subject in hand...



...it may have reached max elevator to maintain the attitude for the speed at that height

This can be a problem with older autopilot types, even the NG aircraft have a rather basic autopilot, carried essentially over from the earlier models, with minor improvements...at least according to a good friend of mine who was an instructor on the NG series.
In fact, I asked him specifically about this.

Can you or 'your good friend' the NG instructor explain this one to me? Are you and 'your good friend' suggesting that the elevator would be maintaining max deflection throughout the scenario described by the original poster?


According to the B737 instructor, the NG autopilot is nothing but a rehashed older Boeing design, originally installed on the B737-200.
Further, the autothrottle system has poor speed control, in some situations.
And, worst of all, during CatIII ops (at least with the NG aircraft that his airline has) there is no roll-out guidance provided, IE: when the aircraft touches down, autopilot(s) disconnect.
Further, and I have been told this last observation by at least two other pilots who have flown the NG type (at two different airlines) that (and I quote)...'the airplane really doesn't touch down all that smoothly, rather it sort of 'flops on', without much finess'.
And, to answer you question specifically, yes, it has been observed by some pilots in the NG aircraft that the elevator has been out of trim in certain situations, because the stab did not trim properly, in the first place.
Shades of the 'ole B707 in this regard, it appears, which had a very similar problem.

We didn't have a problem trying to understand a totally analogue aeroplane There was no advanced technology involved, poor design, poor build quality contributed heavily to poor reliability, we just didn't see the logic of trying to operate a fleet of aircraft that were so labour intensive when the rest of the fleet we maintained.


I find this quite strange, especially the 'reliability and poor build quality' part.
I was told personally that DAL, the largest operator of the type, experienced the best fleet-wide reliability, with their L1011's (versus other aircraft), and they had quite a few other types, at the time.
Lets see, other TriStar operators were EAL, ANA, TWA, CX, GF, SV, RJ, and even small UL.
These other airlines didn't seem to have many problems with their aircraft, so it would appear that 'your' airline either a) could not maintain the airplane properly, or b) a corporate decision was made above your pay grade, to dispose of the type.
Yes, the L1011 is a complicated airplane, alright, however in my twenty eight years of flying it in command, I can offhand remember only a few technical delays...the last three years, only one, a deflated tire.

[Mshamba]

Wouldn't discuss too much about it, same happened to me two days ago in a very normal situation. Descending with my 737 NG in VNAV and LNAV mode, normal operation, on speed, on track, on path - und suddenly and unmotivated AP disconnected, btw with sounding alarm. Re-engaged, normal operation again, didn't disconnect again. Reported to the maintenance on ground, they'll go check but didn't had any idea why it could happen.

Remember, an autopilot might be female, they sometimes misbehave unmotivated... ;)

Or maybe she (AP A) just didn't like the frenchmen, as it happenend soon after reaching french airspace?

[Intruder]

Oversensitive to wine fumes? :)