Flying with a single-engine airplane in case of an engine failure, things are pretty simple. You set the speed for best glide, decide on a best available spot on the ground that you are able to glide to and hope the spot looks as suitable close by as it did up there. If in a climb, nose down, best glide speed and land ahead.
Once you add engines, the operation becomes safer but also more challenging to a pilot when it comes to flying the plane in case of an engine malfunction. Therefore the single-engine operation is a such big part of multi-engine training. If you don’t know what you are doing, it’s very easy to end up upside down and plunging towards the ground. Below an example of how quickly this can happen.
In a twin-engine aircraft the engines are not on the center line like in a single-engine. If the other engine fails, the airplane becomes unbalanced. Thrust is produced only on one side and on the other side the drag is increased. This causes as well a yawing moment as a rolling moment towards the inoperative engine. If the critical engine fails, the effects are even more significant and the airplane becomes more unstable and more difficult to maneuver.
The critical engine is the one, which failure is more critical in terms of controlling the airplane. Critical engines are said to exist only in airplanes that have propellers turning to the same direction. The counter-rotating propellers do not cause this effect, neither do jet engines.
In a propeller the down-going blade produces more thrust than the up-going blade. When the propellers turn clockwise (viewed from the flight deck) the down-going blade is further from the center line on the right engine. In other words the thrust on the right side has a longer arm. From physics we remember that Moment = Force x distance or Torque = Force x Moment arm. From the formula we can see that the right side produces a larger moment. Therefore the left engine is the critical engine. If it fails the larger moment on the right will cause more adverse effects than the left engine would if the right one would fail.
Here’s a video explaining the same thing as well as another factor called slipstream effect:
These effects caused by the propellers are worst when flying at slow speed in climb attitude. That would make the take-off a critical situation for an engine failure. In cruise flight the speeds are higher and the plane is in level flight and the effects of an engine failing are not as drastic as in climb.
We have practiced engine failures in the trainer during take-off roll, after rotation, after lift-off and in cruise. We also practiced flying the traffic circuit, go-around and landing with one engine inoperative (OEI). Practicing these for an hour is a sweaty job and feels like a decent workout! It has been also very enjoyable because it was challenging. You really need to put all your attention into flying the plane, because once an engine fails, a propeller aircraft loses about 80 percent of its’ thrust. With the remaining 20 percent you are supposed to climb to a safe altitude. An example of our aircraft the Diamond DA42. All engines operating climb performance is about 1200 ft/min. One engine inoperative it’s just under 300 ft/min. We added a gear failure to a OEI go-around in one practice, the gear wouldn’t come up, and the climb performance got even worse. You really needed precision to get the plane to climb. And it took a loooong time! Gladly this is quite not the case with passenger aircraft which have much more power available to fly with only one engine.
Another attention demanding action is feathering (a feathered propeller creates less drag) and securing the failed engine. You have to be sure you’re shutting down the failed one, not the other one! First you identify the failed engine. It’s easy to recognize for example by “dead foot, dead engine”. This means that since you need to press the rudder pedal considerably on the operating engine’s side to counteract the yawing effect, the other foot won’t do any work. The failed engine is on the dead foot side. To verify the identification, you can pull the power lever idle on the failed engine. The correct lever is first confirmed and then it is pulled slowly to idle. Once the failed engine has been identified and verified, it can be shut down and secured, which means the power and fuel supply is cut off. We can’t know for sure what caused the engine to fail and securing it prevents a possible engine fire.
All in all we flew 6 flights in the trainer during the past month, four of which included engine failures. We will be practicing the OEI operation also in real life, but the practice won’t be as dramatic as what can be done in the trainer.
We have also gotten a taste of the real Diamond aircraft. I’ve flown two general airwork (stalls, landings, turns etc.) flights so far. The weather has been quite bad lately, the clouds are hanging low, and this has caused many cancellations to our CPL flights which should be flown in VFR. So the first CPL cross-country flight still awaits. But while we’re waiting for that, let’s enjoy the first taste of the Diamond in pictures. Occasionally the skies have been pretty too!