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A simple question that I could not find an answer to:

When is a sailplane a sailplane (from a regulatory point of view), especially a powered one of the TMG (touring motor glider) variety?

EASA has some quite simple definitions, presumably modelled over the similar ones from ICAO, according to which:

‘Sailplane’ means a heavier-than-air aircraft that is supported in flight by the dynamic reaction of the air against its fixed lifting surfaces, the free flight of which does not depend on an engine.

‘Powered sailplane’ means an aircraft, equipped with one or more engines having, with engine(s) inoperative, the characteristics of a sailplane.

‘Touring motor glider’ means a specific class of powered sailplane having an integrally mounted, non-retractable engine and a non-retractable propeller. It shall be capable of taking off and climbing under its own power according to its flight manual.

From this simple chain of definitions, it would seem that, for instance, a Cessna 152 could be classified as a TMG: It can take off and climb under its own power, and with the (non-retractable) engine inoperative it will do free flight sustained by its "fixed lifting surfaces", otherwise known as wings.

Since a Cessna 152 is not normally seen as a glider, I presume that there are more rules that need to be respected before a plane can be called a glider (or sailplane in EASA's classification).

I know that CS-22 contains the specifics, but is there a simple set of rules to determine if an aircraft belongs to the class of (powered) sailplanes as opposed to normal aeroplanes?

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  • $\begingroup$ I'd guess that the "free flight which does not depend on an engine" is where the definition does not apply. The Cessna is capable of gliding safely down to a controlled landing, but cannot be sustained in flight indefinitely by use of thermals and ridge lift. $\endgroup$ – IanF1 May 11 '15 at 19:54
  • $\begingroup$ @IanF1: How long is "indefinitely", for sailplanes? Thirty hours? Ten hours? Two hours? How long can a Cessna theoretically remain aloft under ideal conditions? That's some fuzzy gray area there that really would need precise definition. $\endgroup$ – Nathan Tuggy May 11 '15 at 20:55
  • $\begingroup$ Yes i agree, which is why I put it in a comment :) I guess another way of looking at it is that in the cessna you'd be making a mayday call and looking for somewhere to land, rather than looking for thermals etc. But again that's about SOP rather than what's physically possible. Peter's answer about sink rate is clearly the way to go here. $\endgroup$ – IanF1 May 12 '15 at 5:08
  • $\begingroup$ The Cessna 152 doesn't meet the quoted EASA definition of a TMG. This is because it doesn't qualify as a "class of powered sailplane" because it doesn't have "the characteristics of a sailplane" namely "the free flight of which does not depend on an engine" - It seems to me the quoted definitions are adequate to distinguish aircraft like the 152 from sailplanes, powered sailplanes and touring motor gliders. $\endgroup$ – RedGrittyBrick May 12 '15 at 10:46
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There are indeed several steps between a pure glider and a powered aircraft. The criteria were historically quite simple:

  • Gliders with an auxiliary engine, which is just for getting them home in adverse weather, but not powerful enough for take-off. The propeller is in almost all cases retractable, so their aerodynamics is identical to their pure glider version, apart from a slightly higher minimum wing loading.
  • Motorgliders with an engine powerful enough for autonomous take-off, but with aerodynamics which gives them less than 1 m/s minimum sink with the engine switched off. This minimum sink decides if they are still powered gliders. This would be the TMG category.
  • Powered aircraft with an unpowered minimum sink speed in excess of 1 m/s. The Cessna 152 belongs into this category.

With the introduction of glassfiber motorgliders with their higher wing loading, the minimum sink rule could not be maintained, and manufacturers lobbied for a change in regulations. As it currently stands, CS-22 defines sailplanes to be within its scope if the maximum weight is below 750 kg. Powered sailplanes must not have more than two occupants, weigh 850 kg or less and have a span loading of 3 kg/m² (mass per span squared) or less.

CS 22.71 restricts minimum sink in unpowered flight to 1.0 m/s for single seaters and 1.2 m/s for two seaters.

Please note that suitable instrumentation should also be added for a motor glider to be counted as a sailplane, such as a variometer.

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  • $\begingroup$ Thanks - the requirement for minimum sink rate and wing loading effectively ensures that "normal" sailplanes (powered or not) will be different from other planes. The need for a variometer won't be in the regulations, though, if I understand them correctly. $\endgroup$ – Monolo May 11 '15 at 19:54
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The easiest way to find out for a particular aircraft type is to check its (TCDS) Type Certificate Data Sheet. In it, you will find its certification basis and airworthiness category (CS-22, CS-23, CS-25, CS-VLA etc).

EASA TCDS's are available for free at: https://easa.europa.eu/document-library/type-certificates

FAA TCDS's are also available for free at: http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet

If you search the FAA TCDS database for Cessna 152, you will find its certification basis is CS-23 ("Normal, Utility, Aerobatic and Commuter Aeroplanes") and not CS-22 ("Sailplanes and Powered Sailplanes"). To determine whether Cessna 152 could actually be type certified iaw CS-22 one would have to consult CS-22. I highly suspect that it contains weight limits, maximum energy and/or other limits that would bar the Cessna 152.

For your reading pleasure:

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