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Fuel reserves in a jet

What’s legal, safe, and are they the same?

By J. Mac McClellan

If, as most of us do, you use one of the sophisticated computerized flight planning services for trips in your jet or turboprop, you may have noticed some really huge fuel reserve listed. 

Illustration by Chris Rose.
Zoomed image
Illustration by Chris Rose.

Sometimes the fuel reserve the planning software calculates is larger than the total expected fuel burn for the entire trip. What’s up with that?

Here’s what computerized fuel planning for a trip across my home state of Michigan looks like. It’s a flight we make often in our new Citation CJ4 Gen 2 from Pontiac on the north side of the Detroit metro area to Muskegon County on the coast of Lake Michigan.

The planned distance is 141 nautical miles with a jog up to an intersection north of the direct route. Over such a short distance we’ve given up trying to get higher cruise altitudes from Chicago Center and level off at 12,000 feet. We usually get a direct to destination clearance not long after departure, and the entire flight takes about 30 minutes.

I’ve used for many years and have found its time and fuel calculations to be spot on in every airplane I’ve flown. Other pilots prefer ForeFlight for planning, and its predictions are also reliably accurate.

After looking at the time en route estimate, the next data point I check on the flight plan is usually minimum fuel required. On a typical day with some westerly headwind that minimum fuel required for the trip from Pontiac is around 2,800 pounds. Wow. Nearly half the total fuel capacity of the CJ4 to fly only 141 nm with a reserve at the Grand Rapids alternate, which is only 36 nm away from the destination. How is that possible?

It’s not the en route fuel burn, which is only around 1,000 pounds, that includes 120 pounds for start and taxi. It’s not the fuel to the alternate, which is only around 390 pounds. It’s the 45-minute reserve fuel at the alternate that is the big number. Almost 1,500 pounds. That’s 150 percent of the fuel needed for start, taxi, and cruise to our destination.

Every jet and turboprop pilot I know has a minimum landing fuel they feel comfortable with. For me in the CJ4 I want to land with no less than 800 pounds in the tanks. And if I’m headed to crowded airspace along the coasts or big metros like Dallas-Fort Worth or Chicago I may bump that number up to 1,000 pounds.

So how do the computerized flight planners almost double a reasonable reserve of 800 pounds? The answer is the most conservative reading of FAR 91.167, the rule that governs fuel requirements for flight in IFR conditions.

FAR 91.167 requires enough fuel for airplanes to fly to the intended destination, then on to an alternate airport and then fly after that for “45 minutes at normal cruising speed.” But this rule is open to interpretation.

It’s prudent to always have a diversion plan and fuel in case the destination airport is unavailable for some non-weather-related reason, such as somebody landing gear-up on the runway.First, the rule applies to flight in “IFR conditions.” Does that mean the weather must be instrument meteorological conditions (IMC) or merely on an IFR clearance? FAR 1.1 states, “IFR conditions means weather conditions below the minimum for flight under visual flight rules.” However, Paragraph (b) states that if the forecast for the intended destination—an airport with an instrument approach—is at least a ceiling of 2,000 feet above airport elevation and visibility of at least three statute miles we don’t need an alternate airport. So, with decent weather that meets this exception, an airplane can carry enough fuel to complete the flight to the intended destination and then fly after that for 45 minutes at normal cruise.

That caveat helps a lot on most days, but it’s prudent to always have a diversion plan and fuel in case the destination airport is unavailable for some non-weather-related reason, such as somebody landing gear-up on the runway.

But the real limitation, and the one the flight planning services handle in the most conservative way, is the “45 minutes at normal cruise power.”

On a longer trip where you climb to “normal” altitudes in the flight levels the flight planners come up with a reserve fuel that makes sense. Usually, some number very close to my personal 800-pound minimum comfort level. Why? Because the planning software interprets the “normal cruise” requirement to be fuel flow at the cruise altitude you file for on that specific trip.

At Flight Level 410 or 430 where we would be on any longer trip in the CJ4 the cruise fuel flow would be around 1,000 pounds or a little more. Three quarters of that hourly fuel flow is about 800 pounds. Logical and legal.

But on the short trip from Pontiac to Muskegon with a cruise altitude at 12,000 feet the fuel flow is more like 1,900 to 2,000 pounds an hour. The computerized planners take three quarters of that fuel burn and it becomes the reserve, and that sends the dispatch fuel number soaring.

There is no question the computers are making the most conservative fuel plans. But do they make sense? If I fly 1,500 miles instead of 150 my fuel reserve requirement is 800 pounds instead of 1,500. What factors could blow up my fuel plan over the three and a half hour 1,500-nm trip? Lots. What could change in a 30-minute 150-nm trip? A lot less.

To abide by FAR 91.167, determine a landing fuel minimum that you’re comfortable with and that meets the rule’s requirements. Experience is the best way to establish that number, but while gaining experience it’s best to consult pilots of the same type that you trust.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.

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