747 engine takeoff power

Gord Beaman wrote

Perhaps 'unsafe' isn't the correct word here, my point is that I
feel that it might be unproductive to operate the a/c closer to
it's maximum capabilities just to save some 'wear and tear' on
the engines? I'd think that you're not availing yourself of that
'extra performance' in case of an engine failure at a critical
time.

Gord, I post the following excerpt from the excellent book,
"Handling the Big Jets" by D.P. Davies, the Brit who certified
the B-707 in Britian as Chief Test Pilot for the United Kingdom
Airworthiness Authority.

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The overall safety level of reduced thrust take-offs is
something which bothers some pilots, who believe that the
average exposure to `near critical' take-off conditions
is increased. The proof that this worry is groundless is,
as one would expect of something produced by performance
experts, long and detailed. However, the following is a
brief outline:
When reduced thrust is used for take-off the risk per
flight is decreased because :
(a) The `assumed temperature' method of reducing thrust
to suit take-off weight does so at constant thrust/weight
ratio, and the actual take-off distance, take-off run and
accelerate-stop distances at reduced thrust are less than
at full thrust and full weight by approximately 1 % for
every 3°C that the actual temperature is below the assumed
temperature.
(b) The accelerate-stop distance is further improved by the
increased effectiveness of full reverse thrust at the lower
temperature.
(c) The continued take-off after engine failure is protected
by the ability to restore full power on the operative engines.
Furthermore, although there is inevitably a slight increase
in average risk, this increase is minimised by two factors:
(a) A significant percentage of take-offs are at weights close
enough to R.T.O.W. not to warrant the use of reduced thrust.
(b) The excess margins on lighter-weight take-offs are largely
preserved by the maximum thrust reduction rule.
In any case it is anticipated that more than adequate
compensation will be provided by enhanced engine reliability.
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BTW Gord, just in case you are not that familiar with civilian
jet transports, takeoff thrust does not equate to full throttle.
T.O. thrust is set in acordance with pressure/temperature charts
always with some throttle left to account for the hot/high day.


Bob Moore
VP-21 VP-46
ATP B-707 B-727
PanAm (retired)