Best/Worst Single Engine Performance

[paulsalem]

Cheyenne II has a section in the AFM for single engine take off.

HEADWIND

How does that work? As in you can MEL an engine?

 

[Headwind]

Don't know why anyone would want to do a single engine take off in a Cheyenne II, but like I said there is a section in the AFM. The point I was trying to make is that if it is certified for single engine take off it must have good single engine performance. The Cheyenne II is a Navajo (which had 2 310 HP engines) with 2 620 HP engines.

 

[Lead Sled]

...the Cheyenne II has a section in the AFM for single engine take off.

Huh? I flew one for 3 years and don't remember that in the AFM. (However, the last time a flew one was in 1988. I could have forgotten that section in the AFM, but I tend to remember "minutia" like that.) Its performance was good, but I doubt if it was that good.

I flew a Baron 56TC for about a thousand hours. That airplane had far and away the best single-engine performance of any propeller driven twin - piston or turboprop - that I've ever flown.

Regul8r...
With the possible exception of the Baron 56TC, the single-engine performance in any piston-powered light twin is abysmal. Loss of power is the primary culprit. Remember, normally aspirated aircraft lose power with altitude. An aircraft's climb ability is directly proportional to the amount of "excess" power that it has available vs. what is needed to maintain level flight. For example, if a 200 HP normally aspirated airplane requires 100 HP to maintain level flight it would (at SL, ISA day) have 100 "excess" HP to use for climb. At 10,000' MSL, the engine might only be able to produce 130 HP, leaving it with a 30 HP surplus. This is the reason why turbocharged aircraft perform so well - you would be able to maintain SL power up until you reached the "critical altitude" for the particular engine. In some cases, this can be as high as 18,000' MSL.

This is also why light twins typically perform so poorly on one engine. Take, as an example, a Twin Comanche with two 160 HP engines. If that airplane required, say, 150 HP to maintain level flight it would have 170 "excess" HP to climb with. If it lost an engine, it would have lost 50% of its available power, but with just 10 "excess" HP, it may have lost 95% of its ability to climb. This, of course, will also apply to the Baron and all other light twins. If you only learn one thing from this post, it should be that light twins have two engines because they need two engines.

If you're in a single and you loose the engine, the laws of physics dictate what will happen next. Leaving the gear down until you have no more "usable" runway left only makes sense. It will leave the airplane in much better condition should the engine decide to pack it on while you're still over the runway. I've seen some "hotdoggers" rotate and immediately suck the gear up. A while back, I even heard of a guy who would place the landing gear in the up position as he took the runway. His thinking was that the squat switch would keep the gear from coming up while the airplane was on its takeoff roll and immediately start the retraction process as soon as he broke ground. What a weenie - his practice came to light one morning when he rotated a bit too early and the airplane settled back onto the runway. You don't need a PhD in Physics or Aeronautical Engineering to figure out what happened next.

(By the way, these are usually the same guys who immediately pull the throttle back to climb power as soon as the gear is in the wells. Statistically, that's the time when you’re most likely to have an engine come apart.)

Multi-engine aircraft are an entirely different matter - you want to get the airplane cleaned up as soon as possible. In the case of most piston twins, their ability to maintain controlled flight depends upon the pilot getting the gear up, the propeller feathered, and the flaps up. Failure to this often results in the airplane being turned into a lawn dart.

We tend to take engine reliability for granted, but I believe that this is a very dangerous thing to do. Most light twins simply won't maintain altitude with a windmilling propeller and/or the gear down. The old Metroliners had a rocket bottle in the tail to buy the flight crew enough time to get the gear up in the wells. Auto-feather systems are on most, if not all, turboprop twins. The manufacturers wouldn't put up with the increased costs and complexity of doing this if there wasn't a true need for it. They put this stuff on the turboprops that have significantly better single-engine performance than piston-twins. It really ought to be on the "little" twins, but with the exception of the new diesel-powered TwinStar, no one is willing to pay for it. Like I said, light twins lose up to 90%+ of their climb capability. The figure is probably even higher than that - I've seen several non-turbo'd twins that wouldn't climb out of ground effect and several turbocharged twins that could barely achieve climb rates of 100 fpm or so. But never the less, the point is that twin engine aircraft have two engines because they need two engines and under certain conditions they can maintain controlled flight on one engine IF THE PILOT DOES EVERYTHING RIGHT. You don't have time to dawdle, it has to be done right and right now.

'Sled

 

[Sam Snead]

Don't know why anyone would want to do a single engine take off in a Cheyenne II, but like I said there is a section in the AFM. The point I was trying to make is that if it is certified for single engine take off /quote]

Oh, I WANT to do it. Sounds like fun.

SHOULD I do it? MAY I do it? CAN I do it? Mmmm.

Just bring the power up very slowly and get yourself a very long runway. Wide would help too.