The Boeing 747-400 had a very definite performance and load-carrying edge on the earlier Boeing 747 Classic series, namely the 100, 200, 300 and SP models. This came about because of improved engines that could generate more thrust and also due because of advanced technology that resulted in lighter airframe and system components.
For an ultra-long inter-continental flight of some 14 hours or slightly more, it often happened that a Boeing 747-400 would taxi out for the departure at its maximum take-off weight of 394 625 kilograms. All eight of its fuel tanks would, in all likelihood, be filled to the brim. This would result in a fuel weight, measured at the normal average density for fuel, of some 173 072 kg. Converted to litres, the volume of fuel would equate to 215 558 litres!
Quite understandably, maximum thrust would be required for the take-off, as “balanced field length” and “continued climb performance” requirements after the failure of an engine, would need to be met. At that weight, the aircraft would be right on the limits for a legal and safe take-off. Once the thrust levers had been advanced and the engines had “spooled” up and were operating at the required thrust for that take-off, a glance at the primary engine indications would show that the fan r.p.m’s and the exhaust gas temperatures would be extremely close to the red line limits. Then too, as described in an earlier article, the groundspeed read-out on the navigation display would be increasing closely towards the 195 knot tyre speed limit.
Such take-offs, as described, never came cheaply or without financial punishment. The operation of engines so close to their limits has always been expensive in terms of engine maintenance and fuel usage. The high groundspeeds that tyres are subjected to has also resulted in their costly, earlier than normal replacement.
Now, let’s assume that the very next flight that is done with the same aircraft is a medium range trip of only seven hours or so and that the number of passengers and their baggage is only half of what it could be. The amount of fuel that will be carried is now going to be about half of what it was on the previous flight and the aircraft will then also weigh at least 150 000 kg less. Quite obviously, there would be absolutely no reason to use full thrust for a take-off, as this would be a total “overkill”. The engines would “guzzle” fuel, both unnecessarily and wastefully, and there would be a concomitant shortening of the life of certain of their components. Obviously, the aircraft would now accelerate very rapidly and a far shorter take-off run would result. After rotation, in order to contain the airspeed and stop it from “running away”, the aircraft would have to be climbed at a steeper than usual angle and this would be both alarming and even frightening for many of the passengers.
Taking all of these factors into account, aeronautical and propulsion engineers, together with test pilots from the same companies, started certifying their aircraft for “de-rated” engine thrust situations. On the Boeing 747-400, where most airline pilots were first exposed to this concept, the aircraft was, of course, certified for take-offs at FULL THRUST. However, the aircraft was also certified as if it had a completely different set of engines in terms of lower thrust output. An additional performance manual was provided for operations at DE-RATE ONE, which gave a full 10% reduction in thrust. The DERATE ONE performance manual provided thrust figures and speeds for the take-off and flights that would typically be used for the seven to eight hour flight that was mentioned earlier on in this article .
Now, if that very same aircraft was to operate on an extremely short flight, say for the single hour that it takes to fly from Johannesburg to Durban, the fuel uplift would be a mere 25 000 kg or 31137 litres, and the aircraft would be literally, as light as a feather. The thrust settings and speeds obtained from a certification process done for DE-RATE TWO would then be used. The performance manual derived for this option would give a 25% reduction in thrust.
The ability of being able to safely and responsibly reduce thrust settings for the take-offs and the initial climb-out, result in big cost savings for the operators. The simple and practical way to both understand and to utilise de-rated thrust settings is to view the very same airliner that you happen to be flying, as being exactly the same aircraft, but that it has three different sets of engines. FULL THRUST will give you all that the engine is capable of producing. By the punching of a button on the engine management page, De-RATE ONE may be selected and the engines will now only go as far as producing 90% of their rated thrust. A second punch of the button selects DERATE TWO and the engines will now be limited to producing a mere 75% of their rated thrust.
If, having selected DE-RATE TWO, and whilst operating in the take-off and initial climb regime of flight, a situation occurs or which requires full thrust, then all it takes is the punch of a button or the click of a switch to signal the engine’s electronic management system to reschedule the engines so that FULL THRUST becomes available.
PART 4 of the Boeing 747 Series in the next issue, will be about its fuel jettisoning system…Why there is a necessity for such a system and how it is used.