Hi,please imagine the following scenario :Polar flight from Europe to PANC and the plane encounters a rapid decompression and has to dive down to FL 140 ( at least ) for atmospheric oxygen . . . next suitable airport for diversion is about 1000 nm away. Is the remaining fuel sufficient to reach this airport due to extreme higher fuel-flow at lower level ?I simulated this and my findings are as follows ( PMDG 747-400 ) :Max speed at FL 140 was M 0.68 and now the surprise : the fuel-flow went DOWN from roughly 2.9 Ktons/engine at FL 340 to 2.5 to 2.6 at FL 140.Is this realistic ? I thought always that the lower you fly the higher the fuel consumption - I checked the abnormal proc in the manual but I could not find any solution.Any expert advice ?Karl-Heinz - EDDI/THF " Tempelhof "" The mother of all airports " - Sir Norman Foster

Karl-heinz,It might be normal depending on your weight, but you haven't taking into account that you'll have a considerably less TAS at FL140 than at FL360. In ISA conditions at FL340 the TAS/FF(pr eng. pr hour) is 511/2400kg(280tonnes and .86mach) and with the same weight at FL150(almost same as FL140), TAS/FF is 439/2100(approx)btw, you should descend to 10.000feet or MSA which ever is higher. Martin

Hi Martin,Thanks for the answer - butI have an old manual of the Grand Old Dame " DC10 " and there it says that the specific range at lower altitudes will be reduced by around 30 % ! Per 1000 kgs of fuel from the sixties to the forties of NM ! Or is this only on account of reduced speed ? But the difference between 511 and 439 is only 14 % !?!Are your figures from a real world manual 747-400 or other sources ?Karl-Heinz - EDDI/THF " Tempelhof "" The mother of all airports " - Sir Norman Foster

Karl,let s assume your infos.FL340 at LRC with 4 engines at 340t for 2903 kg/eng/hr mach.859 tas 497 knots all in ISA datas.now about 14000' not pretty sure that s good or you ll carry oxy in huge quantities but i ll write down both infos for you.14000'340 t TAS 434 mach.690 3084 kg/eng/hr10000'340 t TAS 413 mach.647 3080 kg/eng/hrso in this case you need extra fuel and time to make it ...if you give me more infos on your flight then i can generate some scenarios for yousee you