By Bjorn Fehrm
12 June 2015, C. Leeham Co: Earlier in the week we had an interesting interview with Sir Tim Clark, , president and COO of Emirates Airline. We discussed Emirates’ requirement for a twin aisle medium/long range complement to their Airbus A380 and Boeing 777 fleets. The competition is between Boeing’s 787-10 and Airbus’ A350-900. So far the assumptions have been that the 787-10 will be hard to beat on pure costs per seat for mid-range requirements in the 300-seat segment.
The 787-10 seats 323 passengers in Boeing’s old-fashioned IAC three class seating and 331 in our more modern, normalized two class seating with 60 inch angled lie flat in Business and 32 inch economy section. The A350-900 has so far seated 313 seats in the same normalized seating standard. Recent cabin changes by Airbus can now increase that to close to 330 seats. The configuration changes were originally conceived for A350-1000 but we believe Airbus will offer these to Emirates and they will make it into the -900 catalog.
The 787-10 is lighter and would therefore be more effective on fuel but the difference is small, given the A350-900’s more modern engines. So the overall discussion was that 787-10 had found its ideal customer, in need of many seats, a solid mid-range performance and lowest cost. That was until Monday’s interview with Clark.
We approached Clark at the side-lines of the International Air Transport Assn. Annual General Meeting in Miami Beach. We told him other media “understood that Emirates was leaning towards 787-10.” Clark responded: “I don’t know where that’s coming from. If anything, the 10 is not coming up with the thrust requirements that we need. We’re working with Boeing on that, whereas the A350-900 has got bags of thrust.”
How can the Boeing 787-10 be short of thrust? No other customer has raised this issue. We looked into the matter and here is why:
Take-Off from hot airports
Emirates is not a normal airline. Not only do they operate on a scale different from all other airlines but their main hub, Dubai International, can experience ground temperatures of over 45C degrees (113F) on many days of the year. This affects both aircraft and engines. For the aircraft, the density of the air diminishes with 10%, making the air that the wings pushes down to lift-off lighter. To get to the same lift force the aircraft now has to go faster, raising the lift-off speed. Higher liftoff speed requires stronger engines or longer runway. The runways of Dubai International are long, 12,000 feet, but there are other limits that come in like maximum tire speeds. Fix when this happens, lower take-off weight to lower the take-off speed.
For engines, it is worse. The lower air density lowers delivered thrust (turbofans function by throwing air out the back faster than it entered in the intake, lighter air = lower thrust, everything else being equal) but the engine is also suffering internally from the increased air temperature. The higher air temperature enters the intake and ripples through the whole front section of the engine. As it passes the high pressure compressor, it can cause the engine computer (the FADEC) to lower engine rotation speed as the high pressure compressor increases the airs temperature as part of the compression and the air temperature can exceed what the last compressor stages can take. Lower rotation speed means lower compression which saves the last stages from being toasted.
Should the compressor be OK with the higher temperatures, the engine’s turbines will not accept higher temperatures than their so-called flat rating maximum without the engine computer throttling back. This flat rating max temperature is normally set to happen at outside temperature of +15°C over ISA temperature (ISA temperature is the worldwide agreed standard temperature defined at +15°C). Dubai presents +30°C over ISA, i.e. 45°C, on a large part of the year.
This results in reduced thrust from the engines and we have seen that we need more to compensate for the thinner air. The reduced thrust comes from the engine’s computer injecting less fuel in the combustor to save the compressors and turbines from melting. Thereby the turbine driving the fan only produce 60,000 hp instead of over 70,000 hp from a more normal airport.
The end result is that the engine loses power and that the aircraft has to back off on take-off weight as a result. The power loss can be considerable. A 787-10 engine is rated between 76,100lbf (GEnx-1B76) and 78,900 lbf (Trent 1000-K2 or TEN). The critical point in an aircraft’s take-off is the so-called Safety speed point directly after rotation, V2. Here the aircraft has to be able to continue on one engine with a climb-rate of 2.4%, i.e., it shall climb 2.4ft per 100ft forward motion.
We are now down to one engine which due to LAPSE (thrust loss due to forward speed, ref our fundamentals series #2) and the high temperature has gone from 76-78 klbf of thrust to around 55 klbf. Had the Take-Off taken place in Europe, where the high temperature day would have stayed below 30°C, the remaining engine would have delivered over 60 klbf of thrust and the air would have been 5% denser.
The only way to compensate for the thinner air and thrust loss is to haul less payload or to have stronger engines on an aircraft with a bigger wing. An aircraft made for long range flying is designed to take-off at higher weights and therefore have stronger engines and a larger wing. This is why an A350-900 can fly with more cabin and cargo loads on the shorter flights that Emirates plan for than a more mid-range optimized 787-10. The draw-back is that the aircraft has those larger engines and wing; it is therefore heavier and weight costs efficiency. In the case of the A350-900, the difference is not large as the aircraft’s Trent XWB engines are newer than the 787 GEnx-1B and Trent 1000, and their higher efficiency can compensate for the higher aircraft weight.
The problems with a very high thrust requirement for take-offs from Dubai International are well known from the larger aircraft. The 777X has been called “Tim Clark special” for the need for extra large and powerful engines to handle the high temperatures of the Middle East.
It remains to be seen what this means for a more normally optimized aircraft like 787-10. Emirates want good hot performance and good reliability at the same time, a hard nut to crack for engines which are dimensioned for more normal hemispheres.