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Autopilot use on a twin with an engine failure

Ibra wrote:

I never flew piston twins, but I think any MEP on one engine can be degenerated into a SEP with no engine very easily, if the latter is easier to fly in IMC

Once the aircraft has been reconfigured, it is no more difficult than a single as long as the considerably reduced envelope is respected.

Ted
United Kingdom

RobertL18C wrote:

I must be old fashioned but if the AFM has an AP restriction, and most puddle jumper twins have a restriction on using AP OEI, why disobey it

Obvious answer – because it is safer. Just imagine – you are happily cruising along in IMC and mid-route, two hours from your destination, an engine fails. Once the aircraft is cleaned up and in trim again, you now have to figure out where to divert to, liaise with ATC, call up the approach on the EFB which you might never have seen before, figure out how to fly it and what to do if you go missed with very low climb rate, etc – a pretty high workload. Even if there is no turbulence, it will be much safer if there is someone or something who keeps the wings level and maintains the altitude.

I suspect the main reason why the autopilot is not certified for single engine is because it would need a single-engine-mode to limit bank angles and pitch inputs, and it would require quite a bit of testing.

Biggin Hill

My first hand experience at night, far over the sea, with an assymetric DA42 (neat total loss of 1 engine power) is that the autopilot copes very well – almost too well, it took me a while to realise what actually had happened (engine indications all normal, plane flying well but slow)

That’s why there are limitations on OEI IFR diversions, and why the single pilot multi IR check ride requires demonstrating a OEI planned diversion-without an AP.

Oxford (EGTK), United Kingdom

Robert,

I think that that’s a rather rose tinted view of the world. There are plenty of examples of pilots being unable to reproduce in real life what they have, at some point, demonstrated on a test.

EGKB Biggin Hill

Ted wrote:

Once the aircraft has been reconfigured, it is no more difficult than a single as long as the considerably reduced envelope is respected.

I was referring to pulling power completely/partially on the good engine (or just both if one is not unsure which one is down) if for some reason I am unsure of what me/AP/aircraft are doing (say losing it in IMC due to one of the 3 things listed), but Cobalt has answered the question

Of course you are getting a high rate of decent at the cost of few variables to think about as you will do in a SEP…

Last Edited by Ibra at 28 Feb 18:51
Paris/Essex, France/UK, United Kingdom

A functioning AP ought to work in a OEI twin. I say this because it works just fine in a SEP with one wing full of fuel and the other wing almost empty, and a resulting roll of say 5 degrees.

For best speed you should use rudder trim to get the ball back in the middle, and the AP will then fly whatever roll angle is required to hold the heading. That is how an AP works. It runs a proportional+integral (not derivative, AFAIK, so not the classic PID) control loop on the heading error and uses the result to adjust the roll angle. So if say 7 degrees is needed to hold a heading, it will fly that.

It is absolutely not necessary to have wings level to fly a heading. Try it in flight, with a boot full of rudder (not at high speed!).

Administrator
Shoreham EGKA, United Kingdom

If they can’t reproduce the standard and trust an uncertified band aid – with risk of uncommanded loss of control – doesn’t this point to poor currency standards.

Not rose tinted I just would prefer not to buy into the safety benefits of MEP flown by part time private pilots.

Oxford (EGTK), United Kingdom

On Peter’s example there are LOC accidents in the SET world where the operator did not respect fuel imbalance limits.

A slab wing TB20 doesn’t have a fuel imbalance limit I believe.

In ME flying in a side slip with mainly aileron input (possible at SE cruise and well above Vmc) is inefficient and will set you up for a potential fin stall. As speed decreases the AP would roll in more aileron and you would run out of rudder trim or have a fin stall.

Oxford (EGTK), United Kingdom

Peter wrote:

if say 7 degrees is needed to hold a heading, it will fly that.

yes but what about 20 or 30 degrees AoB with no rudder input or what ever the limit AoB actually ends up being at maximum side slip. The point being is there are more failure modes that aren’t fun. I am almost certain these failure modes are not tested in this class of aircraft.

Last Edited by Ted at 28 Feb 19:22
Ted
United Kingdom
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