PW JT8D query

Profile picture for user KabirT

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19 years 9 months

Posts: 7,536

While reading the great article A Dash 7 Mission in the latest AW issue i came across this information, that a 727 PW's burn more fuel on lower altitude than on a higher altitude. My question is what causes this? Is it because the air flow on lower altitude is much heavier and dense than on higher altitude? Same as there is more turulance on lower altitudes compared to higher? Or is it these particular engine type which causes this?
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Profile picture for user murph

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14 years 8 months

Posts: 546

At a lower altitude, the air is more dense. As altitude increases, the density decreases, and the energy required to heat this air will also reduce. The less energy required, the lower the fuel to air ratio required and hence, a lower fuel consumption at a higher altitude. I reckon Deano may be able to go into a bit more detail about this from his groundschool stuff, but thats what I can pick out of after a brief glimpse through my Aerospace Technology 1 Module notes! Engine type naturally has an effect on the amount of fuel burnt, a 727 for example will typically (over a flight) burn more fuel than the latest efficient turbofans. The SFC (specific fuel consumption) is the amount of fuel burnt in kg per kg of thrust per unit time, my notes sadly are in American units, for example a Tornado's RB199 engine has an SFC of 1.76 lbs/hr/lbs (courtesy of the afterburner), compared to the RB211 on a 747 which has 0.363! The lower the better for fuel consumption! ;)
Profile picture for user KabirT

Member for

19 years 9 months

Posts: 7,536

Thanx for the explanation murph. I wonderif the new age engines have such a system that fuel consumption is tried to be kept in a balance at any given altitude? That could probably be a reason of the great fuel efficiency of aircrafts such as the 777ER etc.
Profile picture for user Hugh Jarse

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14 years 8 months

Posts: 192

There is another factor that is considered and that is the fact the thrust is related to temperature difference from the front of the engine to the exhaust (EGT). The engine has a finite temperature it can withstand so if the temperature is higher the difference across the engine is lower as is the maximum available thrust. On the fuel consumption it means that if the OAT is higher the EGT also has to be higher. To acheive this a greater amount of fuel is required to be burnt. There is a little negative to flying higher and that is that, as already mentioned, the air is thinner and therefore the IAS reduces. With a lower IAS the angle of attack has to increase or the IAS must increase. If AOA increases so does the drag and therefore fuel used. To go faster, more fuel is also used. As you can see it is not as simple as going higher to save fuel especially when you factor in winds too.