Air Vanuatu Twin Otter elevator cable snap

[chuks]

I once had an engineer install a hose incorrectly on a Cessna 404, the hose that connected the turbocharger to the inlet manifold. When it popped off, I lost the engine due to the mixture suddenly going far too rich for the suddenly reduced manifold pressure.

This was in a fully-loaded aircraft off a short strip on a very hot afternoon, when I was very lucky to have the hose pop off during extended climb, not too late to abort or right after lift-off.

When I got back to base, the guilty party told me that all I would have had to do was to lean the mixture to get the engine back, that there was no real problem there if I had simply known what to do. That's the closest I have ever come to slugging an engineer, with his blithe notion that I was going to to sit there and twiddle with a few controls to see what might work, in a machine with a highly negative rate of climb due to gear and flaps down and a windmilling prop on a failed engine.

So there you are when the elevator cable on your Twotter goes "Twang!" but you can sort it out no probs by playing with the elevator trim ... on short final or on take-off? Better yet, simply turn around and ask the SLF to run this way or that. These things I would like to see done in practice!

The Twin Otter uses a control lock for the elevator that locks the pilot's controls, just a rod that runs between the control column and the bottom of the instrument panel. That means that the elevator cables are still exposed to stress from winds and jet blast whether the controls are locked or not. The elevator itself is protected from banging around, as are the ailerons and the rudder, but none of the control cables is protected.

[westhawk]

One gets the impression from reading this that this problem essentially only affects Twin Otters?

The twin otters have been under scrutiny for cable wear and corrosion for quite a while. Service bulletins going back for at least 20 years. One big fatal crash that attracted global attention that I know of. Service difficulty reports caused transport Canada to issue ADs requiring cable inspection and maintenance 20 years ago and mandatory life limits later on. With this type, it's been a known issue for which mitigating action was taken.

It's interesting that reports indicate that the broken cable in the Vanatu incident had only 6 months in service at the time of failure. Investigators should soon know whether it was a manufacturing or installation defect if they don't already.

Other types of aircraft are of course affected by cable wear and corrosion. In fact life limits have been imposed on all primary flight control cables for all Aussi registered aircraft. Though US registered A/C currently only have such life limits if imposed by a specific type AD, the Aussie approach could be in our future too. One source of undetectable corrosion cited by the Aussie CASA is the cable terminal ends that were swaged on during the cable assembly manufacturing process. There has been allot pushback over this issue but failed cable terminals continue to be found worlwide.

The simple fact is, the aviation industry has known about this since the advent of steel control cables! For the most part, periodic cleaning, visual inspection and lubrication (where applicable) has been considered sufficient. More recently, field generated service difficulty reports are indicating that undetected wear and corrosion has become more problematic. Hence the CASA reaction.

Some airplanes require a significant number of man-hours to properly carry out inspection and maintenance activities on cable systems. In some instances the cables must be removed from the aircraft to inspect them properly. Then they must be re-installed and rigged. My belief is that many of these cable system inspections aren't being properly accomplished. Many annual inspections are completed in the time it would take just to properly inspect the cables. Mandatory replacement is probably coming to much of the rest of the world outside Australia!



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[777boyo]

The Twotter is a magnificently rugged little aircraft - loved every minute of the 1500 hours I spent on it, and have to say that technical problems were really rare. There were several occasions (not technical related) sitting in the left seat of a 777 in the middle of the night staring into the dark somewhere over an ocean with 10 hours to go that I would have willingly gone back to a Twotter if someone would have paid me the same money . Having said that, the Boeing is a bloody fine piece of kit, too.

[Loose rivets]

Non event - just fly it with the trim wheel.

Luxury!! I dreamed of having a trim wheel when my elevator was solid.

Freelancing on a Shorts out of SEN. I was at about 6,000' in the London TMA.

Thank heavens, nice day and smooth. So smooth in fact that I only became aware of the stiffness of the trim gradually. Then it wouldn't move at all. Gentle pressure on the stick gave the same reaction.

Via my FO I asked London for, and got, a long gentle turn back roughly to SEN with Stansted alerted. I had asked to be routed away from populated areas. Pax strapped in, and our lovely young FA was standing at the door behind me trying to maintain her smile. I briefed on moving the passengers, but stressed that only one must move just a foot or so at a time. My FO went back to eating his sandwiches.

Moving the pax. Okay you, "I wouldn't let that happen" guys. What the hell would you have done?*
I had pressed on the Yoke just about as hard as I dared. After all, if it had gone forward sharply, and then stuck, doing a Neil Williams probably would have done nothing but keep my mind off the metaphorical hole I was in and the smoking hole I would have shortly been in.

Yes, I was gingerly altering the power to make minute changes of attitude but the approach was going to be somewhat more challenging. I had a mild sense of unreality and it made me mindful then, that an order to a passenger might be met with nothing but a blank stare. I thought through a lot of things during that time but despite having thousands of hours in command of medium weight turbo-props I hadn't got my usual bag of tricks open to me. Side-slipping, even on quite large aircraft, had always been a powerful tool for bush-style flying but that always required immediate elevator input, so a non-starter. Just keeping stable was about all I had.

Decent into warmer air freed the controls. I still recall the disappointment of the challenge being stripped away. One is really alive during situations like this.

It seemed the Shed's control run was in the belly. There had formed an emulsion of aviation goo and water - which has frozen over a longish length of the cables. I'd pushed quite hard with the above mentioned caution, but I recall bracing my entire body ready to stop pushing should it suddenly move. While frozen it was solid.


*Flying on trim. There has not been an airline I've worked for where the PF did not use the trim on a moment by moment basis. My first jet transport, the BAC 1-11, had a small gap - 3 degrees I think it was to allow micro switches to cut the Mach trim - and I still recall the clank clank of that hysteresis being taken up. Remember, we hand flew almost everything except cruise - just because we wanted to. When I fist stepped into the ATR for a somewhat impromptu bash at flying it, I reached for a trim wheel that wasn't there. It seemed insane - just as though a major flying control had gone missing.

[funfly]

Flying using the trim might be OK if the elevator is jammed but if the cable has broken it is very possible that the elevator would be able to move around offering a quite different scenario.

[khorton]

Originally Posted by funfly

Flying using the trim might be OK if the elevator is jammed but if the cable has broken it is very possible that the elevator would be able to move around offering a quite different scenario.

Hopefully the elevator will be moving. The idea is that the elevator moves in response to the trim tab movement, and this causes the aircraft to pitch up or down.

If the elevator is locked completely solid, then you only have the tiny trim tab moving, instead of the much larger elevator, so the aircraft pitch response response will be very minimal. And, to further complicate your life, the pitch movement will be in the other sense of normal response to trim movement - i.e. if the elevator is completely frozen, trim nose up to cause a very slow pitch down. Note: this is referring to an aircraft with a trim tab on the elevator. It does not apply to aircraft where a moveable stabilizer is used for pitch trim.

The Twin Otter AFM has an Emergency Procedure to cover loss of elevator control, so the pilots would hopefully have some idea what to do.

From the DHC-6-300 AFM:

Elevator Control Malfunction

WARNING

AIRCRAFT LONGITUDINAL RESPONSE TO ELEVATOR TRIM CONTROL AND POWER CHANGES WILL BE REDUCED. AVOID LARGE AND/OR RAPID CHANGES IN TRIM AND POWER.
SECTION 3 EMERGENCY AND ABNORMAL PROCEDURES

TC Approved

If movement of the control column does not produce a corresponding change in pitch attitude, longitudinal control can be managed by using the elevator trim. Engine power should be used to control vertical speed and airspeed.

Flaps should be extended or retracted cautiously, in increments of 5° or less.

Application of power will usually result in a nose-up pitching moment, and reduction of power will usually result in a nose-down pitching moment.

Extension of flap will usually result in a nose-up pitching moment, and retraction of flap will usually result in a nose-down pitching moment.

[Chu Chu]

Also, I'd imagine that if the elevator was stuck more than a very small amount nose-up or nose-down, the trim tab wouldn't be able to overcome it.

[rileymcdu]

Another related accident (fatal) from 1994, on the west coast of Canada, near Port Hardy. Elevator cable failed due corrosion. The pilot survived, with very serious injuries.

Transportation Safety Board of Canada - Aviation Investigation Report A94P0215

[westhawk]

Another related accident (fatal) from 1994, on the west coast of Canada, near Port Hardy. Elevator cable failed due corrosion.

That's probably the one that prompted TC into action the following year.

Last time I was in Port Hardy, the big issue was how to get thousands of birds off the runway so we could take off. Taxiing down to midfield and blasting them with engine noise and jet exhaust was the locally recommended procedure. It worked too! They even stayed away long enough for us to take off without hitting any.

[RAT 5]

Flying on the trim: I'm sure it would take quite a bit of trial & error and experimentation before I'd make an approach. Is the elevator frozen or is it floating? Which way does the a/c pitch with what trim input? How much trim input = how much change in attitude? What happens with power changes and can the trim control counteract pitch changes caused by power changes? There would be many variables to ascertain and gain confidence with before going near finals. Remember, you still might not know what will happen if you decide to GA. That would also be a manoeuvre to practice at height; and we haven't even discussed pitch changes with flaps yet.
Not quite so easy as first imagined. With a trimming stabiliser, à la B737, some what easier than an elevator & trim tab.

A slight drift about different ways to achieve the same using trim tabs. I was taxying out behind an old DC-9/MD-80 family. The high level stabiliser had elevators, but one was up & one was down. I was totally confused and asked, via ATC, to alert the crew. The reply was "eet's a normal" in accented italian english. Later, in discussion with a type rated pilot he explained, if I can remember correctly, the elevators were floating and pitch control was via the training edge tabs. With airspeed the elevators streamlined and all was good. So I wonder what would have happened if those elevators became stuck. Back to my PPL books about balance tabs & anti-balance tabs etc. etc.

[The Ancient Geek]

If a cable breaks the elevators will be floating and trim will work. This is clearly documented in the POH along with other factors affecting pitch such as flaps. This procedure should be regularly practiced and trained for. We were aware of this and practiced it in Africa but that was a long time ago. I doubt is standards have slipped in the meanwhile but.......

The danger is a cable break at low level with little time to respond, if the aircraft is fully trimmed at all times you have a chance, if you fly with pressure on the yoke it can all go wrong very quickly.

[V1... Ooops]

Originally Posted by Small cog

ISTR from my flying the Twotter many, many years ago that there is an elevator trim / flap / interlink to prevent a recurrence of tailplane overstress during flap retraction.

That's not what the flap-elevator interconnect tab is there for. It is there to reduce the amount of force that the pilot needs to apply to the control column to maintain the same pitch attitude prior to retrimming the aircraft. There is a certification requirement (CAR 3, later SFAR 23) that sets a limit on the amount of force that is needed to maintain pitch attitude following flap extension or retraction but prior to retrimming. Without the interconnect tab, the DHC-6 would not have met that limit.

[V1... Ooops]

Originally Posted by khorton

...If the elevator is locked completely solid, then you only have the tiny trim tab moving, instead of the much larger elevator, so the aircraft pitch response response will be very minimal.

There was an incident many years ago in Canada in which a DHC-6 pilot encountered that exact situation - the elevator was locked solidly in position. It was not a result of an aircraft failure - the pilot was transporting fuel drums that had snow and ice on top when they were loaded into the aircraft. During the long flight, the snow and ice melted in the warm cabin, the water ran down under the floor and froze around the elevator control cable(s).

The pilot was able to land the aircraft successfully by using the trim tab to control pitch and being very judicious with engine power adjustments.

The AFM extract that you quoted earlier in this thread was added as a result of that incident.

[Jetjock330]

25 Years ago, I flew a KingAir C90 that would trim nose up in the cruise and then, disconnect the autopilot with a violent nose up response after an hour of flight or so. Months went by with this snag, changing autopilots and "Please Report Back". GTFS! Ground Tested, Found Satisfactory was the usual response.

Then one day while hand flying the departure, the trim cable snapped in flight. I reduced speed to find the trim speed and fly it from there on wards to landing.

On the ground the authorities got involved, rightly so, following up on each snag in the Flight Folio, and all those about the autopilot problem.

The trim cable became frayed and was rolling onto the autopilot drive pulley and wasn't able to leave the drive pulley as it had wound up on itself. Hence each flight, it got a little worse and worse and eventually snap.

Lessons learn't back then already, that anything wrong with trims, flight controls, stickiness in full flight control check, go straight back to the hangar!

[RAT 5]

Trimming issues can be easily dismissed. Remember Air Alaska DC-9/MD family that spiralled down onto the Pacific after the trim jack broke due to under greasing in maintenance.

[9 lives]

Trimming issues can be easily dismissed. Remember Air Alaska DC-9/MD family that spiralled down onto the Pacific after the trim jack broke due to under greasing in maintenance.

I remember that accident, and the factors, and they should be dismissed relative to discussion of a Twin Otter trim system - the two systems are entirely different. A failed Twin Otter elevator control or elevator trim control will result in the elevator more or less trailing, unless a force is being applied to one of the surfaces. The DC-9 Trim is an all moving horizontal stabilizer, with the trim actuator at the front, which with a failed trim jack screw will mot likely go divergent to an extreme of the trimming range - probably instantly irrecoverable.

The second flap interconnecting trim tab is, as stated, there to minimize pitch control forces with flap position changes. Twin Otter flaps are hugely effective, and this additional system is required.

Control system linkages are known to freeze (in the low temperature sense) in several types. That's a bit of a design oversight that it's possible. Other than that, the seizing of an elevator control in a certain position is much more rare than a one side linkage failure. In such a case, the elevator will generally float free in one direction, and can be controlled against a deliberate out of trim selected by the pilot. But this is not something that most pilots will diagnose, and correctly set up in seconds, so is not an instant fix. I have flown an aircraft on two occasions where a massive pitch control rigging error resulted in flying a circuit while applying only variations in high "push" forces on the control wheel, as opposed to being trimmed, or "pulling" for some of the desired pitch changes. This is counter intuitive, and requires a careful thought, which many pilots could not be expected to get right on short final.