Why don't multiengine aircraft have to demonstrate spin recoverability?

Question

(Building off this earlier question about spin-testing airliners.)

For normal-category aircraft¹ in the U.S. (at least), only single-engine (or, in the case of some gliders, no-engine) airplanes need to demonstrate the ability to recover from a one-turn (or, if the aircraft is intended to be certified for intentional spinning, six-turn) spin. This means that, for instance, the single-engine Cessna 208 (MTOW 3630 kg) has to be successfully recovered from a one-turn spin, whereas the much-smaller Piper PA-34 (MTOW 2155 kg) is exempt from this requirement solely by virtue of having a second engine.

There is nothing about multiengine aircraft that makes them inherently resistant to spinning compared to single-engine aircraft; indeed, if anything, it would be expected to make spins somewhat more likely (think "aftermath of botched or absent pilot response to an asymmetrical engine failure"²). If, instead, the number of engines is being used as a proxy for the size and intended audience of the aircraft (as suggested in this comment on the aforelinked question), then I don't see any reason not to simply use a dividing line based directly on weight (especially since, as noted above, there are many large single-engine airplanes and small multiengine airplanes).³

Given all this, why aren't multiengine airplanes (especially small multiengine airplanes, such as the aforementioned PA-34) required to demonstrate spin recovery like their single-engined compatriots are?

---

¹: Utility and aerobatic airplanes do have to demonstrate spin recoverability even if they have more than one engine.

²: Especially for multiengine tractor-propeller aircraft with wing-mounted engines, where, when flying at very low airspeeds, simply shutting down one engine or losing it to a mechanical failure can be enough to immediately stall its respective wing (due to said wing losing the air previously being blown over it at high speed by said engine's propeller).

³: Granted, even large multiengine airliners can find themselves spinning on occasion, something that (inadvertently) happened to (for instance) the first 307 and the first 737;⁴ however, I accept that an airplane requiring an ATPL and a specific type rating, not to mention a second pilot, is (by virtue of hopefully-good piloting) rather less likely (barring a major structural failure) to be placed in a spin than one requiring merely a PPL, and also that very large aircraft such as these are likely to become debris clouds if spun.

⁴: For the record, both the Stratoliner and the 737 were successfully recovered from their spins, although the former then almost immediately disintegrated in flight due to an overly-aggressive pullout from the subsequent dive.⁵

⁵: Hey, I can nest footnotes!

Answer 1

Because it would impose severe constraints on their design.

Factors such as mass distribution make many twin-engine and multi-engine aircraft inherently resistant to spin recovery, despite the large rudder or large rudder moment-arm that is often present on these aircraft. In many cases these aircraft are prone to flat spins. Also, during the recovery from a nose-down spin, a large aircraft may exceed Vne before pulling out of the resulting dive, or may be overstressed during the pull-out. In the real world, a pilot should avoid spinning a typical twin-engine or multi-engine aircraft, because recovery might not be possible.

"No multiengine airplane is approved for spins, and their spin recovery characteristics are generally very poor." -- from https://www.flightliteracy.com/stalls-in-multiengine-aircraft/

Related questions on ASE:

Is spin recovery possible in an airliner?

Why aren't airliners spin-tested?

Other related links:

https://www.flightliteracy.com/stalls-in-multiengine-aircraft/

https://www.pprune.org/flight-testing/226351-spinning-airliner.html

https://www.airlinepilotforums.com/hangar-talk/6370-spinning-airliner.html

google "spin recovery twin" or ("multi") for much more

Answer 2

The most common response I've been given is that the inertia developed with so much outboard mass cannot be overcome by the flight controls. That said, the Duchess was extensively spin tested and found to be recoverable in established spins. My guess is recovery would be unique to each design based on power plant location and mass, rudder design, and left vs. right hand spinning (excluding counter-rotating designs).

Answer 3

I can only speculate on why the FAA does not require it since I don't know their reasoning. Here is why I don't think it is necessary:

The primary tool to stop an aircraft from spinning is the rudder:

Power to idle
Ailerons neutral
Rudder opposite the spin until rotation stops
Elevator forward to break the stall

^{(What is the Beggs/Mueller emergency spin recovery technique?)}

In a single engine aircraft, the spin recovery needs to be demonstrated because the rudder might otherwise not be sized to be able to stop the spin. It would only be large enough to provide lateral control during takeoff and landing as well as coordinate turns in flight. Full rudder will never be used except for spin recovery, so this becomes the dominating factor.

On multi-engine aircraft, the rudder must be sized to compensate asymmetrical thrust in case of an engine failure. This limits the takeoff performance via $V_\text{mcg}$ (Are there any aircraft where minimum-V1 is limited by Vmcg?). The rudder will therefore already be sized large enough and spin recovery is no longer the dominating factor determining the required size of the rudder. I assume that is why it does not need to be demonstrated.