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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
    Commented Jan 11, 2019 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
    Commented Jan 11, 2019 at 4:19
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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
    Commented Jan 11, 2019 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
    Commented Jan 11, 2019 at 11:21
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    $\begingroup$ @GdD - I consider it one of the best flight-dynamics-for-pilots resources I've ever encountered. I have yet to find any errors in it. $\endgroup$
    – quiet flyer
    Commented Nov 7, 2022 at 23:08

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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
    Commented Jan 11, 2019 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
    Commented Jan 11, 2019 at 15:57
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    $\begingroup$ @FreeMan Would performance not be better at MSL than at 5000ft? $\endgroup$
    – TomMcW
    Commented Jan 11, 2019 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
    Commented Jan 12, 2019 at 13:19
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    $\begingroup$ I remember reading in a popular aviation magazine years ago that statistically, you were more likely to perish in a twin-engined aircraft with an engine out than you were in a single-engine aircraft with an engine out. It all centered around lack of proficiency in singe-engine ops in a twin, mostly near the ground. $\endgroup$
    – RetiredATC
    Commented Nov 7, 2022 at 17:42
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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 assumption is flawed. Generally, twin engines are there to be able to haul more payload into the air, not to be resilient to engine failures.

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To clear up confusion; most light twins do not climb on one engine were certified as such:

FAR 23.67 provides the single‐engine climb performance requirements to airplane manufacturers for FAA certification of multi‐engine aircraft.

For aircraft with a maximum weight of 6,000 lbs., or less and a VSO of 61 knots or less: The single‐engine rate of climb at 5,000’ MSL must simply be determined with the—

  1. Critical engine inoperative and its propeller in the minimum drag position
  2. Remaining engine(s) at no more than maximum continuous power
  3. Landing gear retracted
  4. Wing flaps retracted
  5. Climb speed not less than 1.2VS1

• The rate of climb could be a negative number – meaning a descent • There is no requirement for a single‐engine positive rate of climb at 5,000 ft., or any other altitude.

For Aircraft with a maximum weight of 6,000 lbs. or less, and/or VSO more than 61 knots: If certified before February 4, 1991: the single engine rate of climb in feet per minute at 5,000’ MSL must be equal to at least .027 VSO2.

If certified after February 4, 1991: maintain a steady climb gradient of at least 1.5 percent at a pressure altitude of 5,000 ft. with the —

  1. Critical engine inoperative and its propeller in the minimum drag position
  2. Remaining engine(s) at no more than maximum continuous power
  3. Landing gear retracted
  4. Wing flaps retracted
  5. Climb speed not less than 1.2 VS1

NOTE Do not confuse the date of type certification with the airplane’s model year.

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    $\begingroup$ OP’s question is “why’ This doesn’t seem to answer that. $\endgroup$
    – Jim
    Commented Nov 7, 2022 at 18:14
  • $\begingroup$ FAR 23.67 was already quoted here. Also, using a > formats text to indicate that it's a quote and makes it much easier for others to read. $\endgroup$
    – FreeMan
    Commented Nov 10, 2022 at 19:29

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