The autopilots I mentioned are two-axis, and in most light twins without rudder boost for asymmetric flight. When operating OEI asymmetric yaw will vary in relation to power output of the live engine and IAS, it stands to reason that a two axis AP (without rudder servos) will only be operating in a narrow performance envelope with a risk of uncommanded disconnect, and a lot of aileron input in certain circumstances when the disconnect occurs.
I have never come across an AP in a GA MEP that isn’t placarded against use when OEI, albeit have only experience in eight MEP types.
The sideslip is used to take advantage of the reduced drag from the horizontal component of lift with an MEP/MET banked up to 5 degrees towards the live engine in the climb OEI. In reality the optimum bank is around 3 degrees for most GA MEP (a function of lifting surface between wings and tail fin rudder), arguably the regulatory 5 degrees for establishing Vmc during flight test might accommodate stubbier wings. The small bank generates some horizontal component of lift, which reduces rudder input, sideslip beta on the tail fin (less induced drag), and eliminates fuselage drag. The ball is in fact tilted slightly towards the live engine.
I hope the above clarifies how this is beyond the capabilities of a two axis AP of the Mitchell design philosophy, let alone a rate based S-Tec.
Not flying correctly asymmetrically will increase Vmc (potentially a lot), reduce SE ROC and service ceiling.
The following video is excellent and a very digestible primer on asymmetric principles of flight.
RobertL18C wrote:
The PA-30, PA44, C310, BE55 and BE76 all guilty as charged, with some types enjoying Vmc training fatalities in low triple figures. The PA34-200 may be immune due to low power and counter rotating engines.
PA30 crashed mainly due to instructors shutting down engines at 50-100’ AGL during training, nothing is wrong with the aircraft. The PA44 has a counter rotating engines just like the PA34-200 so why one is good and the other is not? I would think that in a twin you are mostly venerable between 0-300’ after that if the pilot is on the ball all ends fine.
Many/most of the accidents are due to pilot errors which also include over loading the aircraft.
An illuminating example is the 2008 fatality of a PA44 in Croatia, D-GZXB. There is a good BFU report but I can’t find the link.
The many fatalities during Vmc demonstrations were actually due to flying at 5,000’ AGL. What occurs in a normally aspirated twin, is that as the live engine loses power as altitude increases, Vmc reduces, until it goes below Vs0. You now have the hapless instructor approaching the stall with considerable potential yaw from the live engine, and unfortunately this results in a flat spin. MEPs are wing loaded, so their spin characteristic is very different to the more fuselage loaded single engine.
So in fact classic Vmc rollover accidents following an EFATO are not necessarily the aerodynamic reason for Vmc demo accidents, a significant proportion are flat spin outcomes. The FAA finally introduced Vsse, discussed in the BFU report, to avoid this. Unfortunately in the case of D-GZXB the instructor was either not familiar with Vsse, or ignored ATO practice in the demo.
https://aviation-safety.net/wikibase/dblist.php?AcType=PA44
Here is the German language report, but there is an English one.
The PA30 is a very fine MEP, but it required that its Vmc red line, and Vsse, be increased to ensure a satisfactory margin of safety.
I have fond memories of a night flight to Penticton in a 1964 PA30 41 years ago, and counting the mountain beacons following breaking out of an NDB approach before descending further.
This is again a very one sided view of the risks.
I am not talking about the EFATO situation. Of course you do that hand flying, indeed it would always be lower than you would engage autopilot.
Yes, in the EFATO there is some benefit to banking towards the operative engine for best performance, but actually my experience of training is that it just gives the student “another thing to think about and get wrong” and is probably best left to the performance limiting situation, either as described by Dave in the desert or in a really poor performing training aircraft.
I would worry that someone encountering an EFATO in reality might well misremember and bank the wrong way, and also, in the effort of trying to get it right, forget something much more vital. Most MEPs that are in live use will climb away perfectly happily wings level.
Regarding Robert’s point about 2-axis autopilots, this falls straight into the trap I was talking about above. Most MEPs will cruise on one engine at maybe 50kts above Vmc and will climb at several hundred fpm. There is no point in emphasising Vmc worries except in the climb or in the approach.
The theoretical reason for not using the autopilot because of an increase in Vmc or a decrease in service ceiling is so outweighed by releasing enough brainpower to think about management and diversion that it’s in a different league.
I wonder if this discussion underlines the need for scenario based training? I suspect that some of these attitudes arise from the training assumption that engine failures always occur at 200’ after take-off? This is certainly not my experience (so far); all my failures have been in the cruise, except one which only manifested itself when I reduced power in the circuit. That has biased me towards thinking about how to manage the flight after the failure (again, refer back to the video at the beginning) rather than about how to maintain control, which is only an issue in about the first 30 seconds of flight.
Robert, as a matter of interest, how many times have you had a real engine failure when in the cruise, especially in IMC?
Robert, as a matter of interest, how many times have you had a real engine failure when in the cruise, especially in IMC?
Am a complete neophyte, have never had an engine failure in an MEP in flight. And that is despite operating some well used equipment, including the very benign potato head Apache (and yes it can climb OEI if flown correctly). We must compare notes on maintenance facilities. 
I do brief precautionary shutdowns during MEP training, and our ATO still expects a shut down and re start, although no longer required under EASA.
Am not sure how much of my MEP time is actual asymmetric but could be around 400 hours? Recall that on an IR most training exercises carry out EFATO and OEI approaches and diversions (albeit the transit is two engine once ToC), so quite familiar with OEI flight characteristics in the cruise.
Less of the “personal” stuff please. Keep the discussion technical.
First of all, happy new year.
Regarding currency & emergency procedures, one thing that I was taught is that to survive an emergency in a twin, the point isn’t to do things quickly, it is to do them right. For instance, you have to feel the yaw before feathering an engine, you have to take the time to double check everything before pulling those levers, etc.. (there is a nice article on code7700 about that).
I’m a fairly low time twin pilot (~250 hours MEP) but having flown a maintenance-challenged twin (now squawk-free for 87 hours and counting), I had a few interesting events. One shutdown (fuel imbalance leading to fuel starvation because of a broken cross feed valve), one partial failure (clogged injector) above the Sierras, another fuel imbalance (we rebuilt the fuel system after that one) and a couple of other weird things. Despite all this, to me the utility that a larger platform provides is worth the additional “risk”, real or imagined.
Plus, twins are twice the fun :)
RobertL18C wrote:
quite familiar with OEI flight characteristics in the cruise
This, to answer Peter’s point, is not personal, it is an honest quest for knowledge and understanding.
I think that the difference between operating OEI in a training environment, knowing where you are going to “divert” to, and being thoroughly familiar with all the airfields within 100 miles is almost completely different to having a real engine failure over unfamiliar territory.
I am Biggin based, and prior to that Fairoaks, Gatwick and Heathrow. I have had engine failures resulting in diversions to Madrid, Rheims Prunay, Aalborg, Bergen, Leicester and Cumbernauld (that I can remember). I would say, and I am ready to be corrected, that that involves a quite different set of skills to the IR training and testing environment.
In the IR training world, you can afford to focus on flying skills, because you don’t have to get weather, find documentation, brief on facilities, brief on approaches, fuel plan, navigate safely over unplanned territory, manage and secure systems or liaise (in any meaningful sense) with ATC.
Again, and I am sorry to keep referring back to it, but the video at the head of this thread is an excellent example of what it’s really like. It’s all about information management and aircraft systems management, not about stick and rudder skills.
And it is that stuff that private pilots are so badly prepared for in the JAA/EASA IR syllabus. The Americans do it a little better, because they are freer to introduce scenarios, but even there there is no enough focus on decision making in the real world.
The main reason is that most PPLs spend no time in the sim doing scenarios.
When I have got students through the IR, I plead with them to come back to have scenarios thrown at them, just to practice the TDODAR thing. I, personally, go to an IRE who specialises in scenario training, and I go twice a year, because I think it so valuable. Recently I have had a massive fuel leak (which meant shutting down an engine as opposed to recognising a failure; a much more realistic scenario), a fire, bird strike, asymmetric flaps, no compass, no gyro, double engine failure on departure and so on. Things that really happen. Sudden engine loss at 200’ does happen, of course, Dave had one, but it accounts for maybe 1% of reality and 100% of training!!
So, in the real world, you want that autopilot in as soon as safe and practical (especially in IMC) so you can focus on the management tasks. Yes, there is a risk associated with using the autopilot, but it is very small compared to the risks of poor management due to overload.
Peter wrote:
Why exactly would an autopilot not work in OEI?
I suspect because the edge conditions in the OEI certification requirements are actually fairly complicated, and too difficult for an old analog device to cope with.
Peter wrote:
one flies something like 7 degrees wing down
That is not true, fairly close to wings level gives best performance, and but NO more than 5 degrees is allowed to be used in the demonstration of Vmca. AoB beyond 5 degrees for straight flight may allow flight at lower speeds but is not allowed in the certification for good reason. Control and Performance are two different things.
An engine failure makes the aircraft yaw and roll towards the dead engine. You counteract the yaw with rudder and any remainig roll with aileron to keep the aircraft flying straight of sorts.
But an aircraft with asymmetric thrust does not really fly straight.
With wings level and no yaw/turn, the ball is in the middle, but the nose is pointing a bit towards the live engine and the aircraft is in a slip towards the dead engine, i.e, the relative airflow comes a bit from the side of the dead engine. But the ball is in the middle.
While perfectly fine, this is not the most efficient configuration – you can improve performance by counteracting the slip and banking a bit towards the live engine, and reducing rudder pressure a little bit. The ball will be a bit towards the live engine, but aircraft performance will be improved slightly.
However, there is a maximum bank angle beyond which this no longer makes things better, but worse quickly, because of the extra lift required as AOB increases. You want to stay well away from that point, hence the typical “bank up to 5 degrees into the good engine for maximum climb performance”.
When twins get certified, the manufacturer can write up to 5 degree bank into the POH, which has the result of a lower Vmca and a better single engine rate of climb. I don’t know if they are allowed to take that ibto account for certification figures, though.
In any case, know your POH applies.
