Why does the Airbus A300/310 series only have 1 trim switch per yoke instead of 2?

[EXDAC]

Originally Posted by Uplinker

Two rockers don't actually give any particular advantage, because if one jams, the other won't do anything, even if it still moves.

Let's say the left switch jams up (AND). When the pilot releases the AND input the right switch will return to center and trim will stop. The other switch did something - It stopped the trim.

Pilot doesn't notice the left switch is stuck and provides a second AND input. Trim responds as commanded. Again the other switch did something. It allowed the required trim change.

Then pilot need ANU trim and tries to move both switches down. He now notices that the left switch is in the wrong place and can't be moved and uses alternate trim. In this case the other switch did not do anything.

All the types I have worked on had two trim switches. I don't know the reason for changing to a single switch.

[MechEngr]

My thinking isn't that one jams and cannot be moved.

It is that one of them is acting unreliably and not returning to center. In the case of separate toggles this is noted immediately and can be felt by the pilot. With a single toggle the failure of one switch is masked by the ability of the other to return them both to center via the shared toggle. Rather than giving a warning that one of of the switches is on the road to failure, a single toggle masks that.

[dixi188]

On the subject of A300 pitch trim systems.
Had a situation one night where one of the pitch trim systems was inop. and had been deferred IAW the MEL for 24 hours, the previous day.
The spares were not available to repair the system but the MEL allowed both systems to be inop for 7 days, so maintenance pulled and tagged the circuit breakers for the other system leaving us with no autopilot and only manual pitch trim. We were going to Cairo from Stansted, (about 5 hours). The captain said, "No Way". I asked which system was the good one and cut the ties on the CBs and reset them. Off we went.
We never got an answer as to why one system could be inop for 24 hours but both could be inop for 7 days.
(The MEL times may be slightly wrong but the gist of the event is as above)
Dixi.

[Uplinker]

I am going to pretend that I didn't just read that you unlocked some CBs without authorisation and then flew the aircraft.
edit, Oh just noticed you are an LAME, not a pilot.

Originally Posted by EXDAC

Let's say the left switch jams up (AND). When the pilot releases the AND input the right switch will return to center and trim will stop. The other switch did something - It stopped the trim.

Pilot doesn't notice the left switch is stuck and provides a second AND input. Trim responds as commanded. Again the other switch did something. It allowed the required trim change.

Then pilot need ANU trim and tries to move both switches down. He now notices that the left switch is in the wrong place and can't be moved and uses alternate trim. In this case the other switch did not do anything.

All the types I have worked on had two trim switches. I don't know the reason for changing to a single switch.

my bold

My thinking isn't that one jams and cannot be moved.

It is that one of them is acting unreliably and not returning to center. In the case of separate toggles this is noted immediately and can be felt by the pilot. With a single toggle the failure of one switch is masked by the ability of the other to return them both to center via the shared toggle. Rather than giving a warning that one of of the switches is on the road to failure, a single toggle masks that.

No, because the aircraft will warn of a pitch trim problem if one of the switches underneath the rocker fails either mechanically or electrically.

Originally Posted by Vessbot

The particular advantage has been given multiple times in this thread already: there is no trim runaway with a single jam. And the aviation industry seems to agree with the advantage outweighing the disadvantages, being that most airplanes have this design.

Well, I guess that all the Airbus designers and all the Airbus test pilots in Toulouse are wrong then !

Odd that they would make such a mistake - one that Pprune was able to spot within minutes - because their A320 family is better than the conventional manual pitch trim system.

(yes, I am type-rated on both).

[Vessbot]

Originally Posted by Uplinker

Well, I guess that all the Airbus designers and all the Airbus test pilots in Toulouse are wrong then !

I'm not privy to all the detailed aspects of the decision (but it sounds like you are, from your offhand mocking tone - so I'm curious to hear all about them) but on first blush from knowing the basics about airplanes in general, yeah I'd say they are.

[MechEngr]

Originally Posted by Uplinker

No, because the aircraft will warn of a pitch trim problem if one of the switches underneath the rocker fails either mechanically or electrically.

It cannot detect a mechanical failure. If the toggle cannot be moved, the control system cannot detect it. If the toggle becomes mechanically disconnected from the switches, it cannot be detected.

It can detect a differential electrical connection, but not a failure that has not reached the point of a differential electrical connection. It can certainly not detect that one switch is not responding with the same amount of reaction force when joined with a single toggle that would be detected in comparing two separate toggles.

The point you are making is that after the failure has occurred and it is too late to do anything about that failure, the system can notice some aspects of that.
The point I am making is that with two separate toggles the pilot has the chance to detect that the switches need to be replaced before there is an in-flight failure.

[InSoMnIaC]

Originally Posted by MechEngr

My thinking isn't that one jams and cannot be moved.

It is that one of them is acting unreliably and not returning to center. In the case of separate toggles this is noted immediately and can be felt by the pilot. With a single toggle the failure of one switch is masked by the ability of the other to return them both to center via the shared toggle. Rather than giving a warning that one of of the switches is on the road to failure, a single toggle masks that.

on the A300 if one switch is stuck in one position due to an internal failure, the action of moving the pitch trim ‘dual lever’ will automatically disengage the pitch trim, requiring pilot action on the overhead panel to reengage it. This automatic protection essentially requires the switch design that we see where both switches are mechanically linked. Because a boeing type, dual rocker design would cause constant pitch trim disengagements

[MechEngr]

Originally Posted by InSoMnIaC

on the A300 if one switch is stuck in one position due to an internal failure, the action of moving the pitch trim ‘dual lever’ will automatically disengage the pitch trim, requiring pilot action on the overhead panel to reengage it. This automatic protection essentially requires the switch design that we see where both switches are mechanically linked. Because a boeing type, dual rocker design would cause constant pitch trim disengagements

That is not detecting that there is a problem developing; that is detecting that a failure has already occurred. I don't see that this detection requires them to be mechanically linked. Separated toggles would act the same way as they are effectively joined by the thumb of the pilot. There will always be a delay between the operation of independent switches, even if joined together by a single toggle and so there will be a timer to determine how long they can be out of synchronization before deciding there is an error.

If one cares to, look at the problem of switch debounce. A typical switch makes dozens of separate contacts as it is moved into position, opening and closing dozens of times before finally settling down over a period of hundreds of microseconds - ages of time in a digital system. Two switches operated in parallel have no chance of being synchronized over such a period of time.

See https://www.eejournal.com/article/ul...bounce-part-2/ if curious about debounce. It is a lengthy read, but there are decent diagrams and links to other studies on the topic.

[Uplinker]

Originally Posted by Vessbot

I'm not privy to all the detailed aspects of the decision (but it sounds like you are, from your offhand mocking tone - so I'm curious to hear all about them) but on first blush from knowing the basics about airplanes in general, yeah I'd say they are.

Airbus employ the best designers, engineers and test pilots, and are so successful that their order books are full many years into the future.

I am absolutely certain that they know far more about designing control systems and manufacturing aircraft than I do !

[dixi188]

There is more than one way to skin a cat.
I don't see a problem whether two switches are mechanically linked or are independent. They both give the same level of safety.
In the Sim we would get a trim runaway and hold the trim wheel to dis-engage both systems, then re-engage one at a time to see which system was at fault. Sometimes it would be a dual failure.
As for reliability, I don't remember there being a problem on the A300. I don't know about the later Airbuses, A320 and on.

[Uplinker]

Airbus designed-out and removed pitch trim switches altogether in the A320 family.

The aircraft auto-pitch-trims; even in normal manual flying, and the PF simply uses the pitch trim wheels if there has been significant computer failure*

*which is an extremely rare occurrence.

[Vessbot]

Originally Posted by Uplinker

Airbus employ the best designers, engineers and test pilots, and are so successful that their order books are full many years into the future.

I am absolutely certain that they know far more about designing control systems and manufacturing aircraft than I do !

​​​​​​You misunderstood, I'm talking about the design of trim switches and not fawning over your favorite airplane maker like we're comparing music bands or Game of Thrones factions.

[Uplinker]

No, I am simply saying that Airbus are not some tin-pot back yard chancers, that's all. Some on this thread are implying that Airbus have made a mistake or a cheap fix, but that won't be the reason.

Airbus do employ the best designers, engineers and test pilots - as Boeing used to do back in the day - and they wouldn't use a single physical rocker without good reason. The A300 also had a pair of FBW spoilers, so Airbus were designing and producing FBW on that airframe too.

Originally Posted by MechEngr

It cannot detect a mechanical failure. If the toggle cannot be moved, the control system cannot detect it. If the toggle becomes mechanically disconnected from the switches, it cannot be detected.........The point you are making is that after the failure has occurred and it is too late to do anything about that failure, the system can notice some aspects of that.
The point I am making is that with two separate toggles the pilot has the chance to detect that the switches need to be replaced.........

Correct. But the pilot can - and will - detect any mechanical issues with the rocker - whether it is mechanically sticky, or sticking, or graunching, or not actuating both switches underneath.

[Vessbot]

Originally Posted by Uplinker

and they wouldn't use a single physical rocker without good reason

Neat - but we're like 3 back and forths in this exchange, and you still haven't said what this "good reason" might be - or elaborated on "Two rockers don't actually give any particular advantage."

[dixi188]

The A300 was designed in the late 1960s / early 1970s. A lot of the systems followed on from the Concorde. The ADI and HSI were almost identical. I don't know what type of trim switches the Concorde had but the design of both aircraft seems to have worked.

[Uplinker]

Originally Posted by Vessbot

Neat - but we're like 3 back and forths in this exchange, and you still haven't said what this "good reason" might be - or elaborated on "Two rockers don't actually give any particular advantage."

Well, actually I have offered some possible reasons; Check posts 6, 19, 24, and 33 again.

What I keep trying to say is that us here not knowing the reason does not mean there is not a good reason !

Airbus - hugely successful and with vastly more knowledge, engineering and design qualifications than us - will have identified good reason(s), otherwise they wouldn't have designed it that way !

With my engineering background, and 20 years flying Airbus and Boeing, and other types on the line; I can make an educated guess at reasons if you want ?

A single rocker is:

Simpler mechanically.
More robust.
Larger and easier to operate.
Easier to seal against dust ingress.
More reliable.
Has no operational disadvantage over two rockers any more, (with modern electronics and fault detection systems).

Some have said that two rockers allow a pilot to know when one switch is becoming faulty and intermittent, but the same is true of Airbus' single rocker and their associated monitoring electronics.

Back in the day when the pitch trim was a simple switch controlling an electric motor, and aircraft were electrical and hydro-mechanical only; two switches wired in series and two rockers were a sensible way of preventing trim runaways over a single basic switch arrangement, since any basic electrical switch can potentially collapse internally or weld on.

But with electronic fault monitoring; two separate rockers have become unnecessary, since there are still two switches under the single rocker, and any single switch failure or "stickiness" can be detected and mitigated by the electronics or the pilots.
.

[EXDAC]

Originally Posted by Uplinker

Back in the day when the pitch trim was a simple switch controlling an electric motor, and aircraft were electrical and hydro-mechanical only; two switches wired in series and two rockers were a sensible way of preventing trim runaways over a single basic switch arrangement, since any basic electrical switch can potentially collapse internally or weld on.

I don't dispute that simple series switch configurations may be used on some aircraft but I don't know of any. More typically each of the switches in the pair controls a separate function and both function states have to agree for trim to run. E.g. -

MD-80 uses an electric trim motor controlled by two switches. One switch controls the motor drive relays and the other switch controls the brake solenoid. Trim requires motor to be powered and the brake to be released.

MD-11 uses hydraulic trim motors controlled by two switches. One switch controls the solenoid valve directional spool and the other switch controls the solenoid valve shut-off spool. Trim requires both spools to be energized for trim to run.

[Uplinker]

OK fair enough, but the exact circuit details are not germane. The point is, two electrical switches are used, not one, and both need to be electrically and mechanically functional, to move the elevator trim.

But the Airbus designers, engineers and test pilots decided that separate rockers were not required on their A300/310 and that both switches could be operated by one larger rocker

[Musician]

Originally Posted by Uplinker

But with electronic fault monitoring; two separate rockers have become unnecessary, since there are still two switches under the single rocker, and any single switch failure or "stickiness" can be detected and mitigated by the electronics or the pilots.
.

Actually, fault monitoring is simpler with the single toggle, because then both switches have to move at the same time, and if they don't, that must be a fault.
With separate switches, the pilot can move just one, so the system can't decide as quickly.
So for a FBW aircraft, it makes sense to design for electronic fault monitoring, even if that means the pilot can no longer feel the underlying switch go bad.

[Someone Somewhere]

Originally Posted by Musician

Actually, fault monitoring is simpler with the single toggle, because then both switches have to move at the same time, and if they don't, that must be a fault.
With separate switches, the pilot can move just one, so the system can't decide as quickly.
So for a FBW aircraft, it makes sense to design for electronic fault monitoring, even if that means the pilot can no longer feel the underlying switch go bad.

This is very standard in industrial safety. Unless both channels move simultaneously, you disregard the input. Sometimes raise a fault that needs to be manually cleared, other times just wait for the next time both switches actuate together.

If you want to take it further, you have two switches, each of which has two contacts that must operate together. That's common for e.g. presses, where you don't just want to verify that the button was pressed, but that the user actually is clear and wanted to press it. Door release on some trains, too.