Re: Delta ordering flex takeoffs for MD-88s?

From:         Andre Berger <aberger@innet.be>
Date:         04 Aug 96 16:44:48 
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[This is a piece of private e-mail, posted here with kind permission
of the author as I think it contains some excellent details.  Karl]

If Delta orders reduced thrust take offs on their jets in 1996, they are
more than 15 years behind the rest of the industry. Poor PR. But let me
clarify a few points:

- Reduced thrust take off does NOT reduce fuel consumption. The TSFC (thrust
specific fuel consumption) does not vary significantly between a full take
off and derated take off. A take off and climb with reduced thrust on a B737
costs about 50kg fuel MORE than at full thrust. This is because *reduced*
climb thrust is set in function of *reduced* take off thrust, keeping the
aircraft longer at inefficient lower altitudes (higher TSFC in climb).

- Reduced thrust take off SIGNIFICANTLY reduces engine wear and tear,
especially on the newer big fan engines. On the CFM56 (B737, A320/A340) a
single full thrust take off equals about 30 (thirty) reduced take off cycles
regarding wear and tear on the engine (data from our engineering
department). The difference on our JT8D-15A powered B737-200's is much smaller.

- Reduced thrust take off makes MORE noise under the initial climb flight
path and is definitely NOT a noise abatement procedure. On the contrary, a
typical noise abatement departure is a FULL thrust take off followed by a
thrust reduction to a VERY LOW value (well below max climb thrust) when over
the microphones and resuming normal operations when well clear of these. The
exact procedure depends on where the microphones are, but usually they are
not at the departure end of the runway. Anyway, you would hardly hear the
difference between a full or reduced thrust take off.

- It is to the captain to decide to perform a reduced thrust take off or
not. The day that company management makes that decision, I quit flying
(BTW, I am a so called *management* pilot).

- Reduced thrust is used for *Go Around* also. Not for noise, fuel economy
or engine wear or tear reasons but simply because some birds are so
overpowered that it becomes *dangerous* to put full thrust when the aircraft
is lightly loaded. Ever flown a 737 with two 23.5K engines at 36T with full
thrust? You simply will bust all altitudes below 5.000ft if you attempt to
do so. In modern airliners, go around thrust is set i.f.o. a desired rate of
climb.

- Reduced thrust take off (if possible and not in all circumstances) is
safer than a full take off because of the added safety factors (there is
always excess thust, even with the *assumed temperature* calculation
method). IN ADDITION, if an engine fails, you can still set full thrust on
the operating engine(s). This is not a performance *requirement* but an
option you have if need be.

- The only problem area I see is the rejected take off. Reduced take off
calculations do not increase safety margins in case of a decision to abort
near V1. I think they should.

Interesting that the Delta MD80 engine failure was uncontained. The things
are -supposedly- certified so that this kind of engine failure remains
inside the engine nacelle. Blades should not penetrate the cabin. Is this
why the FAA remains silent about the issue?

Kind regards,

Andre Berger
aberger@innet.be