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While perusing the See How it Flies website, I came across this alarming statement regarding multiengine general-aviation aircraft:

You must not allow yourself to think that just because airliners can climb with an engine out, your favorite light twin can climb with an engine out.

It is legal to operate a light twin with anemic or nonexistent single-engine climb performance. In such cases, engine failure at low altitude is perhaps the most critical situation that arises in general aviation with any appreciable frequency. Like a single-engine aircraft with partial power failure, you need to make a forced landing.

Given that the main point of having multiple engines is the ability to maintain flight if one of them kicks the bucket in midair, why are GA aircraft with more than one engine allowed to be incapable of maintaining single-engine flight when all this does is double (or triple, or quadruple) the likelihood of a forced landing subsequent to an engine failure?

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    $\begingroup$ There is an old saying for light twins: "the second engine is there to get you to the accident site faster" (or variations of it)... $\endgroup$ – Ron Beyer Jan 11 at 4:04
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    $\begingroup$ Note that the first paragraph refers to climbing on one engine; many (most?) GA twins can at least maintain cruise altitude on one engine, so takeoff is the main problem. Even if they can't, half power gets you a lot more range to pick a good forced landing site (maybe even an airport) than zero power would. $\endgroup$ – StephenS Jan 11 at 4:19
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    $\begingroup$ Also, redundancy isn't the only reason for multiple engines; reliability goes down as power goes up, so it's obviously better for larger aircraft to have two small, reliable engines than one large, unreliable engine. $\endgroup$ – StephenS Jan 11 at 4:25
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    $\begingroup$ If someone has an answer, could they consider in their answer the statistical possibilities of one out of two engines failing, three out of four engines failing with modern GA aircraft. That might be too broad of a response. I would think with engines from the last twenty years, reliability and predictability of failure has increased the Mean Time Between Failure. $\endgroup$ – gwally Jan 11 at 4:38
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    $\begingroup$ I'm not sure what that site is, part blog, part other stuff, I wouldn't consider it authoritative. $\endgroup$ – GdD Jan 11 at 11:21
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It is legal to operate a light twin with anemic or nonexistent single-engine climb performance.

No, and technically it would not be certifiable as airworthy as per § 23.67 Climb: One engine inoperative. Aircraft that were certified at a different point in history may subscribe to different regulations.

(a) For normal, utility, and acrobatic category reciprocating engine-powered airplanes of 6,000 pounds or less maximum weight, the following apply:

(1) Except for those airplanes that meet the requirements prescribed in § 23.562(d), each airplane with a VSO of more than 61 knots must be able to maintain a steady climb gradient of at least 1.5 percent at a pressure altitude of 5,000 feet with the -

(i) Critical engine inoperative and its propeller in the minimum drag position;

(ii) Remaining engine(s) at not more than maximum continuous power;

(iii) Landing gear retracted;

(iv) Wing flaps retracted; and

(v) Climb speed not less than 1.2 VS1.

In other words if you can get to ~5000 Ft. you need to be able to maintain a nominal climb in a light twin.

The regulation for aircraft over 6000 lbs. is similar as well depending on where you draw the line for "light". There is also more information on how the testing occurs in this AC.

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    $\begingroup$ What if you're taking off in your light twin from LAX (or other airport at/close to MSL) and are still well below a 5,000' pressure altitude when one engine goes pop? After all, 5000 feet gives you a few minutes of glide time to get yourself organized. 500 feet, not so much. $\endgroup$ – FreeMan Jan 11 at 15:14
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    $\begingroup$ @FreeMan I was thinking the same thing. Strangely it only seems to be covered by the regs for aircraft over 6000 LBS where your single engine climb out needs to match your 50ft climb out as per sub section (f) $\endgroup$ – Dave Jan 11 at 15:57
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    $\begingroup$ @FreeMan Would performance not be better at MSL than at 5000ft? $\endgroup$ – TomMcW Jan 11 at 21:47
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    $\begingroup$ A very important bit in that regulation is that it applies to aircraft with $V_{s0}$ more than 61 knots. Well, guess what, many twins out there have $V_{s0}$ (flaps down stall speed) of exactly 61 knots, so the regulation does not apply to them. Which suggests the answer should, actually, be yes. $\endgroup$ – Jan Hudec Jan 12 at 13:19
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Given that the main point of having multiple engines is the ability to maintain flight if one of them kicks the bucket in midair,

I think that concept is flawed. Generally, twin engines are there to be able to haul more payload into the air.

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