The more my experience grows with twins, the more total hoop I hear talked [not referring the posts above!]. I really don’t think anything is more dangerous or needs some kind of macho heroics in milliseconds before doom, but the enevelope of tolerance of sloppy handling and poor drills is reduced. Ergo, people with poor currency can run themselves into problems. Whether it is a single or twin, most of it is about selecting and maintaining an appropriate attitude (whether visually or on instruments).
I would think Autopilot and other limitations are more to do with the certification tests and range of scenarios vs evidence case rather than practical issues in many aspects – I have been involved in similar cases to do with avionics vs temperature in the Middle East!
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,
Strictly speaking not, because a SEP always has a stalling speed of 59 knots or less in the landing configuration. Even a DA42 har a stalling speed higher than that.
Airborne_Again wrote:
Strictly speaking not, because a SEP always has a stalling speed of 59 knots or less in the landing configuration
True, there are lot of limits on performance and design (e.g. MTOW, VR, VS0, VS1, wing span…) to be able to be able to certify it as SEP
My point an MEP with no engines = SEP with no engine = Glider, all will fly like a heavy/fast glider above VS0 and will stall bellow it and have some Vg, or is there something special (apart from fast RoD) when flying MEPs with no engines? someone who can fly/land M20K dead stick, will not be able to do the same on a DA42?
My understanding most MEP will have closer flying stability/envelope criteria to SEP (even 59kts VS0 could go up to 65kts? or MTOW higher than 6000lbs?) but these are certification limits/tests for aircraft classes, for sure regulators need them (especially 60kts/35kts stall marks and 600kg/6000lbs for weight) but for flying sometimes they make sense, sometimes they don’t (I personally, did not feel much difference flying the same aircraft between TMG/Micro/SEP classes, the advertised cost was different tough 
)
I suspect the issue is two fold. Firstly, I doubt a great deal of flight testing is done with the A/P on one engine. Then in theory the A/P would need to be tested with what a pilot might do on one engine with the autopilot engaged (however unwise) including changes in power, and in configuration, both of which could cause the A/P to unexpectedly disengage. The A/P manual would otherwise say A/P only to be engaged with the aircraft trimmed and in stable flight and to be disengaged before any significant power changes or changes in configuration.
I think every MEP I have flown has the no A/P limitation when asymmetric and would tend to agree with Fuji’s comments above; it probably requires a deal of extra test flying and a resultant 47 extra pages in the AFM defining a new set of boundaries under such circumstances. The DA62 says “No AFCS” but I observed someone capably use it the other day when asymmetric on an IR revalidation flight. That said, I probably would have intervened if he had left the A/P on during the climbing turn towards dead engine at Vyse (Blue Line). There’s a whole pile of slipping going on when assymetric and my sixth sense tells me that the A/P’s sixth sense doesn’t have quite the same survival instinct. For sure, S&L – switch it on. At all other times, beware.
IMHO it is a difficult problem to solve with the GA instrumentation, or it is simple to solve if you accept some basic restrictions where the pilot provides input. If you can solve the former, why not go the whole hog? i.e. fly by wire…
Ibra wrote:
My point an MEP with no engines = SEP with no engine = Glider, all will fly like a heavy/fast glider above VS0 and will stall bellow it and have some Vg, or is there something special (apart from fast RoD) when flying MEPs with no engines? someone who can fly/land M20K dead stick, will not be able to do the same on a DA42?
My point is that you can’t expect to deadstick to a runway. The likelihood of a survivable off-airport landing/crash is roughly proportional to the energy dissipation which increases with the square of the landing speed. There is a reason why (real) SEPs are certified with a max Vs0. You can’t compare off-airport deadstick landings in a SEP to the same thing in a MEP just as you can’t compare an off-airport deadstick landing of a glider (which usually is a non-event) to the same thing in a SEP.
This is a point that I feel people miss in the discussion of SEP vs. MEP engine-out safety, particularly in the take-off situation.
Airborne_Again wrote:
This is a point that I feel people miss in the discussion of SEP vs. MEP engine-out safety
Yes, most comparisons refers to the number of engines only, I think the trade-off is more complicated than that in the case of engine failure: in a MEP you have 1/ high MTOW & VS0 with no engines vs 2/ hard time flying with one full or partial engine that you then compare to a SEP 3/ low MTOW & VS0 with no engine
So I agree,
If 1/ >> 3/, I will be cautious about any statement regarding the safety benefit of 2/, especially when not current
If 1/ == 3/, then I only see gains from 2/ with best case: you are current in OEI and worst case: fly it like a SEP with no engine
If I want a proper comparison, 2>1 is not the right way to do it, I would probably go for MTOW(SEP)/[VS0^2(SEP)] vs P(OEI)*MTOW(MEP)/[VS0^2(MEP)] where P(OEI) is probability of me shutting down the good engine, crashing on OEI, stalling the rudder…
In an attempt to better understand the certification requirements for autopilots I found this:
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23-17C.pdf local copy
The section on autopilots starts at page 250 and runs for about 10 pages. It is a recent document, but the autopilot section is “essentially equivalent” to the previous guidance from 1965! My reading of it suggests that all phases of flight and performance expected of a part 23 airplane, needs to satisfied, unless a limitation is placed on the autopilot.
My guess is then that the manufacture/STC holder, in many cases has simply taken the approach of prohibiting OEI flight with the A/P, using the limitations section of the POH, rather than open a can of worms regarding testing and liability.
In contrast an acceptable means of compliance for part 25 flight guidance systems is over 100 pages… https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25_1329-1C.pdf local copy
From here
The restriction may be due to the A/P not being certified in this condition, which in turn may be due to the A/P leading to a loss of control (probably insidious).
Most puddle jumper A/Ps are two axis, so no rudder input (absent a yaw damper which will not be certified for OEI). Trying to control yaw with roll and pitch (remember 2 axis), may and will result in a fin stall due to excessive Beta sideslip at speeds well above Vmc. The fin stall will result in a nice crisp departure of controlled flight, usually to inverted nose down.
Would an airline or commercial operation ignore an AFM/POH restriction just because on the few times the operator ignored the restriction no harmful consequences resulted?
Single crew MEP flying requires currency, the A/P is not designed to deal with an engine failure/asymmetric conditions.
