TBM 850 Fuel Consumption and Block Time Analysis for a
Typical
Short-Haul Flight
by Omer Majeed
Specific
Range Solutions Ltd.
Summary
A study was undertaken to evaluate
the effect of cruise power settings on total fuel consumption and block time
for a short-haul flight of a light single-engine turboprop aircraft.The effect upper winds, air temperature and
altitude were also addressed.The
aircraft chosen for the study was the Socata TBM 850 which is powered by a
Pratt & Whitney Canada PT6A-66D engine.The mission defined for analysis was a relatively short IFR flight
carrying four passengers from Ottawa MacDonald-Cartier International Airport
(CYOW) to Toronto Buttonville Airport (CYKZ), a distance of 182 nautical
miles.The forecast weather for flight
planning purposes was obtained from NAV Canada's website for a 15:00 UTC (10:00
am local) departure on January 22nd, 2009.The aircraft performance calculations were based on the TBM 850
Pilot's Operating Handbook1 (POH) per Ref. 1.
The baseline flight profile
involved a maximum power climb at 130 KIAS to FL300, followed by a cruise
segment at that altitude and a Continuous Descent Approach at 150 KIAS and a
descent rate of 2000 fpm to the destination.Fuel consumption varied from 382.5 lb for the Long-Range Cruise power
setting to 402.2 lb for the Maximum Cruise power setting, the block times
varied from 87.4 min. to 78.9 min.By
pulling the throttle back to the Long-Range Cruise setting, 19.7 lb or 4.9% of
fuel is saved, but the flight will have an 8.5 min. or 10.8% longer block time
compared to the Maximum Cruise setting.The reason for the small difference in the amount of fuel burn is due to
the relatively short length of the cruise phase (115.9 NM).
Compared to calculations at ISA conditions
at FL300, the fuel consumption and block times using the predicted upper wind
and temperature data are significantly higher for all cruise throttle
settings.This is due primarily to the
relatively strong winds from the northwest at altitude varying from 300o
at 97 knots at the start of the trip (CYOW) to 310o at 63 knots by
the end of the trip (CYYZ), although the marginally colder temperatures (ISA-5oC)
at altitude also contribute to slightly higher fuel burn per the POH.
A check on the calculations was
performed based on published data. Per Ref. 2, it is stated that 392 lb of fuel
would be consumed over 225 NM at the Long-Range Cruise setting with NBAA IFR
reserves (100 NM), zero wind, ISA and a 658 lb payload, 392 lb of fuel would be
consumed.This compares to 334.2 lb of
fuel consumption calculated over 182 NM at the Long-Range Cruise setting at
FL300 at ISA with 45 min IFR reserves and 4 passengers aboard.Extrapolated to 225 NM, 413.2 lb of fuel
burn is estimated using the SRS' methodology, therefore the calculated result
is in reasonable agreement with published performance data.
Flight planning was also performed
for a cruise altitude of FL240 using forecast wind and temperature data to
evaluate the effect of the lower altitude on fuel burn and block time.While the air is denser at lower altitude,
thereby generating more drag, the predicted winds were slightly more favourable
compared to FL300: 300o at 86 knots above CYOW and 300o
at 62 knots above CYYZ.The fuel burn
varied from 399.4 lb for the Long-Range Cruise power setting to 416.2 lb for
the Normal and Maximum Cruise power settings2, the block times
varied from 93.7 min. to 75.9 min.Expressed in percentages, the Long-Range setting burns 4.0% less fuel,
but takes 23.5% more time compared to the Normal and Maximum settings.The True Air Speed is significantly reduced
at that altitude with the Long-Range throttle setting, therefore explaining the
significantly greater block time.However, the FL240 cruise altitude profile is faster than the FL300
profile: 75.9 min versus 79.5 min for Normal Cruise power and 75.9 min versus
78.9 min for Maximum Cruise power.This
is primarily due to the time saved by avoiding the additional climb to FL300 at
130 KIAS and by avoiding additional descent from FL300 at 150 KIAS.
To conclude, the most
efficient flight profile based on the forecast winds and temperatures in terms
of minimum fuel consumption is Long-Range Cruise at FL300 with a total fuel
burn of 382.5 lb and a block time of 87.4 min.The fastest profile is Normal/Maximum Cruise at FL240 with a block time
of 75.9 min and a total fuel burn of 416.2 lb.
Table 1 - TBM 850 Fuel Consumption, Block Times & Specific Range for Short-Haul Flight
Notes: 1. Fuel consumption for taxi, take-off and descent at 150 KIAS flight phases were estimated due to the data not being available in the POH. 2. The Normal and Maximum Cruise power settings are identical at FL240 and ISA-5oC.
