From: zari@bolzano.math.ethz.ch (Alain Zarinelli)
Subject: Twin aircraft (was: Re: Twin Beechcraft crashed in Hinckley, IL)
Organization: Swiss Federal Institute of Technology, Zurich
Date: Thu, 10 Sep 1992 14:11:19 GMT


In article <1992Sep10.041119.18763@ncrcae.ColumbiaSC.NCR.COM>,
hutton@ncrcol.ColumbiaSC.NCR.COM (ed Hutton ext. 6488) writes:

>           [...]            you are FOUR times as likely to suffer a
> fatal accident if you lose an engine in a twin engine airplane than 
> if you lose the engine in a single engine airplane!
>                               [...]
> Surprised at this, I asked a couple of OLD crafty twin drivers at the
> local air strip. I got two responses that stuck with me.
>
> "The good engine carries you to the scene of the crash."
>
> "A twin has two engines because it needs them both."
>
> I can think of several reasons why I might want to fly a twin.
> An extra engine is not one of them. The single engine performance
> numbers I have read are pretty weak.


     Am I missing something here?

     If a single engine aircraft loses its (only!) engine, it just
descends to the ground; controlled, but irrevocably. On the other hand,
if a twin engine aircraft loses one of its engines it can perfectly well
continue flying (or should I say slipping? In any case not _GLIDING_!)
to the nearest airport and perform an -- "absolutely normal" (one's
trained for that!) -- single engine landing...

     Sounds perfectly easy, doesn't it!? Unfortunately things aren't
that simple. A normal flight has more elements than just cruise and, for
_light_ twins (MTOW < 5.7T, operating under FAR 23), the crucial moment
of every flight (at least with respect to an engine failure) is the
transition from the TKOF roll to the first climb segment. The two
engines _together_ give the power necessary for TKOF and climb; here Ed
is perfectly right. If one engine dies shortly after TKOF the pilot's
got problems ( :-( ): the remaining power simply doesn't suffice to get
the aircraft into a climb in the DIRTY CONFIGURATION (gear and flaps
down, propeller of dead engine unfeathered). This is the crucial
difference (among others) between commercial aircraft (MTOW > 5.7T,
operating under FAR 25) and light twins: if a commercial aircraft loses
one engine during TKOF, it _MUST_ still be able to climb -- EVEN IN THE
DIRTY CONFIGURATION. There are two well-defined speeds which (at least
officially) don't exist for light twins: v_1 and v_2. v_1 is the speed
above which the TKOF can't be aborted any more as there's not enough RWY
remaining. v_2 is the speed the aircraft needs to climb safely with
flaps extended, gear down and an unfeathered propeller. v_2 is, for most
of the light twins, unreachable. For example: a Piper Seneca with 2
people and fuel for 2 hours can't even get near v_2... To make things
even more complicated there is one "critical" engine if the two engines
turn in the same direction (if they turn in opposite directions neither
is critical): during climbs the down-spinning propeller-blade gives more
thrust than the up-spinning. The way the two engines are mounted on the
wings gives asymmetric thrust vectors and thus an angular momentum
tending to twist the aircraft. This is the reason why the losing of the
critical engine during TKOF is somehow the "worst case".

     The moral: Ed,

     I don't really understand your statement "FOUR times as likely to
suffer a fatal accident if you lose an engine in a twin engine airplane
than if you lose the engine in a single engine airplane". I don't
believe this as far as cruise engine failures are concerned.  Perhaps
you can elucidate?!

     I subscribe to the statements of the "OLD crafty twin drivers" but
with the restriction to the TKOF part of a flight.

     The "extra" engine _IS_ a good reason to fly a twin (IMHO); you
just have to keep in mind _WHERE_ you may encouter _WHAT_ problems --
and stay proficient in the emergency procedures...

						Alain.
--
Alain Zarinelli                                            (PPL-CVFR/NVFR)
zari@math.ethz.ch                                                         
zari@astro.phys.ethz.ch               "If God had meant man to fly ---
zari@czheth5a.bitnet                   He would have given him more money"
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