What is the most fuel-efficient flight profile for a small plane?

Question

If you take a short one hour flight in an A320, the flight will usually climb to around 25 - 30,000ft and then descend into its destination. The actual cruise at 30,000 will often only be a matter of minutes. The reason for this, I understand is it's the most economic (fuel sparing) flight path.

What is the most economical vertical flight path to take when flying a small plane (say a 152 or ultralight) for a flight duration of around an hour?

I recently tried the "A320" approach by climbing to 10,000ft (the flight ceiling) and then switching to idle. But since small planes often don't have a great glide ratio, including the one I used (C42), I believe that flight was less economical than usual, it took a very long time to get up there at a fairly high power setting.

Answer 1

Few aircraft in the class of the 152 have a high enough service ceiling for reduced fuel burn at altitude to matter much, plus unless you have oxygen you're limited to 12,000 feet as a general rule.

Likely the biggest factor to save fuel with these aircraft is to avoid high throttle once you're clear of the airport and high enough to reduce throttle. You'll burn a lot less per mile traveled at 70% than you would at 100% throttle, and reducing to 50% will save even more -- and because drag goes as the square of speed, flying, say, 20% slower saves more than 20% on fuel burn, so your overall "fuel mileage" is better when you throttle back.

Second is to ensure you're running the mixture as lean as EGT and head temperature (depending on available instruments) will allow. Back in WWII, correct mixture management added more than 20% to the range of a P-38.

Answer 2

It depends on a lot of factors so it's hard to make generalizations. But for such a short flight, in most GA aircraft the minimum fuel burn profile is almost definitely flying the minimum legal altitude. Or the altitude with the most favorable winds, if there is a significant difference.

Here is a plot of the range profile of a Cessna 152 with standard tanks:

You can see that, at minimum cruise power, the range is maximum if flying at sea level and only decreases with higher altitude.

On the other hand, here is the same plot for a Cessna 152 with the optional extended range tanks:

You can see a modest increase in range up to 4000 feet and a decrease afterwards. The much longer flight allows for the modest efficiency gain of flying higher to make up for the extra fuel burned in the climb.

This is about a nine hour flight, though. It takes basically the whole 9 hours of fuel savings in cruise to make up for the fuel burned in the climb.

Note that these charts are based on a maximum performance climb. A somewhat faster airspeed climb could possibly give some fuel savings. But most POHs do not contain the information necessary to analyze fuel burn with arbitrary climb speeds.

This is based just off of fuel economy. Keep in mind that many maintenance costs depend on the number of hours spent in flight. Saving a few dollars on fuel isn't very helpful if it costs more than that in additional maintenance costs.

Answer 3

Zeiss Ikon covers a lot of the points about operation of the physical aircraft but I think one of the most critical choices in light aircraft efficiency can be:

Winds Aloft

Your most economical flight path can be the one that takes the greatest advantage of winds aloft and/or avoids large headwinds. With modern flight planning apps like Foreflight you can generally see these options in realtime if you have programed your fuel burns in.

If you flying VFR you can take advantage (within safe altitudes) of choosing your cruse altitude to optimize tail winds and avoid head winds.

For example lets say I wanted to fly from KDXR to KDDH right now, I can climb up to 11,000 feet where Im nice and high and close to my service ceiling but if i stay a bit lower at 5000 and take advantage of the tailwind Ill burn 2.9 less gallons of fuel and get there at the same time. Its worth noting this is my 55% lean mixture profile which is the most efficient for this aircraft. If time is of no factor to you this will generally always yield the lowest burns. The spreads are comparable at my other throttle positions with higher burns throughout.

(Source: Own Image)

Answer 4

Light aircrafts have special fuel consumption characteristics because of their design, dimensions, power plant and other factors. Small aircraft especially those used for private uses should have details on the performance factors among them being glide ratio and the climb rate. The recommended operating environment is at a height of 3,000 – 5,000 feet, and the most preferable climb and descent gradient should be a moderate gradient. More to do’s include the flight planning, management of the fuel and referring to the Pilot’s Operating Handbook (POH) for better performance.