The number that is not fixed

A 747's thrust rating is the number most often quoted about it and the one most often wrong. It is not a fixed property of the aeroplane. On the -400 and the types before it, the same certified type could be operated at different thrust ratings, and which one it carried was a decision an operator made and could change.

The ratings, by variant

Boeing's own airport-planning documentation lists the certified rated takeoff thrust options:

747-400	Rated takeoff thrust	747-400ER	Rated takeoff thrust
CF6-80C2B1F	57,900 lb	CF6-80C2B5F	62,100 lb
PW4056	56,750 lb	PW4062	63,300 lb
RB211-524G2	58,000 lb	RB211-524H8-T	65,500 lb

Two things jump out. The three engine makers certified to broadly similar thrusts for the -400, within about 1,250 lb of each other - engine choice on the -400 was genuinely competitive, as the engine pages set out. And the -400ER ratings are all substantially higher, because the -400ER exists to carry more weight and more fuel, and that requires more thrust to fly the same speeds.

The 747-8 then removed the choice almost entirely: it has a single engine option, the GEnx-2B. Flexible thrust rating is a phenomenon of the -400 and earlier types, not of the modern one.

Why a higher rating costs engine life

Within a single engine family, the higher-rated versions are not different engines. They are the same gas path asked for more.

A higher rating is obtained with a higher throttle setting, more fuel and higher rotational speeds. The immediate consequence is temperature. Engines are limited by exhaust gas temperature, and the margin between the actual EGT and the redline is what absorbs a hot day, an ageing engine and a slightly worn compressor. Spend that margin in advance and there is less left to give.

This shows up in the certification data as a flat-rated temperature - the outside air temperature below which the engine can hold its rated thrust constant. The Pratt & Whitney PW4000-94 family demonstrates the pattern clearly: the higher-rated versions have lower flat-rated temperatures. The lower ratings hold constant thrust to 33 °C; the highest 62,000 lb rating holds it only to 30 °C. For an aircraft whose whole purpose is long over-water sectors, where the diversion and alternate planning is sensitive to performance in warm conditions, that is not a trivial difference.

The same arithmetic then continues into the shop. An engine spending its life closer to its temperature limit accumulates damage faster, and a 747-400's ultra-long sectors - where engines routinely recorded double-digit engine flight hours per cycle - left very little room for waste. A new engine of a given rating has more margin than the same engine after a shop visit.

Changing the rating

A thrust rating change is a ground maintenance action, and for the 747's PW4000 it is accomplished through the electronic engine control programming plug. This is not an informal adjustment: FAA airworthiness directive guidance treats a rating change made this way as an event with its own conditions, requiring the applicable cyclic limits to be applied for the period in which the change was made, and distinguishing engines that were new or overhaul-complete at the time from those that were not.

That regulatory attention is itself informative. An AD does not bother tracking a number unless changing it has consequences for the hardware - which is exactly the point. A rating change is closer to re-certifying the engine's operating envelope than to adjusting a preference.

What it means to the operator

The decision is a genuine trade-off between three things:

• Hot-and-high performance. Routes through the tropics, or from hot airports, depend on being able to hold thrust and to meet takeoff and climb requirements at weight. A lower-rated engine may simply not make the published figure.
• Takeoff field length. The airport-planning documentation organises its takeoff field length tables by aircraft weight and by thrust setting, which is the clearest evidence that thrust is a variable the operator actively manages on each departure rather than a fixed constant. Reducing takeoff thrust where the runway allows it lowers wear and fuel burn.
• Engine economics. Time on wing, shop visit cost and the interval between removals are the largest single maintenance line on a 747, and the rating chosen has a direct effect on all three.

Because the change is a ground action through the engine control, it also interacts with fleet planning: engines could be moved between aircraft, and an operator could effectively standardise a rating across part of a fleet for maintenance simplicity or differentiate it for operational reasons.

The weight context

Thrust rating only means something relative to weight, and the same planning documentation that lists the ratings also defines the weight envelope the 747 works within: maximum design taxi, landing and takeoff weights; operating empty weight; and maximum design zero fuel weight, from which maximum payload is derived. Those definitions are set out on the weight and balance and loading page, and they are the frame of reference for every performance number the type has ever advertised.

Related

• Engines - the five engines and the aeroplanes they sold
• Pratt & Whitney PW4000 - the -400 mainstay
• General Electric CF6
• Rolls-Royce RB211-524