Direct operating costs and power settings

Is low fuel flow really cheaper?

January 1, 2013

turbine mentorFor a pilot coming from a piston airplane—or even a heavy twin—the fuel flows experienced in light jets can be shocking. Especially at lower altitudes, a panicked look often crosses new jet pilots’ faces when they perform the mental calculation first from displayed pounds per hour into gallons per hour, and then to dollars per hour. It’s understandable many new jet pilots end up flying lower altitudes at drastically reduced power settings, feeling somewhat better seeing the correspondingly lower fuel flows.

What is often not considered, however, is that fuel costs only represent a portion of the total direct operating costs to fly a jet. Lowering power to reduce fuel flow will usually reduce the cost of fuel on a given trip; however, the correspondingly increased time in the air will cause a rise in nonfuel direct costs.

Many light jets are flown on engine and airframe maintenance programs in which the owner essentially prepays maintenance at a flat hourly rate, rather than risk sporadic large shop bills. A typical light jet owner might pay about $100 per hour for a parts program, $125 per hour for a technician labor program, and another $125 per hour (per engine) for an engine program. This means that for every hour the aircraft is in the air, the operator must write checks totaling $475—the nonfuel direct operating cost of the aircraft (ignoring depreciation, to which assigning a dollar value can be tricky).

If that aircraft is operating at a relatively inefficient altitude of 17,000 feet, the pilot may be faced with the choice between flying at maximum-cruise thrust (MCT), and seeing a fuel flow of nearly 960 pph, or pulling power back to long-range-cruise (LRC) with a reduction in fuel flow to about 670 pph. A reduction in fuel flow of nearly 45 gallons per hour is certainly attractive, but it will be accompanied by a decrease in speed of 70 knots, as TAS drops from 320 to 250.

At a relatively low fuel price of $5 per gallon, we see the MCT total cost per hour is made up of the $475 for non-fuel costs, plus 143 gph (960 pph) times $5 per gallon, or $1,190 per hour. Dividing by 320 KTAS to calculate the total cost to fly one nautical mile, the result is $3.72. At LRC, the total cost per hour is lower at $975 per hour, but the corresponding lower cruise speed yields a higher cost per mile of $3.90. Contrary to what the pilot may be thinking, it’s actually costing him more money out of pocket to fly at long range cruise.

Even if the pilot is topping off at a large metropolitan airport at a pricey $9 per gallon, the relatively higher fuel cost may barely make LRC the winner, but only by one cent per mile. Given the drastically lower cruise speed and increased time in the air, even the most miserly of pilots wouldn’t likely accept 70 knots of speed loss to save one penny per mile, again rendering MCT the more practical choice.

Neil Singer is a Master CFI with more than 7,200 hours in 15 years of flying.