Ever had a conversation with Air Traffic Control and they have asked you ‘What is your endurance? Usually the answer is a thumb suck in anticipation of a clearance to come in and land, but have you ever wondered what exactly endurance means, and how you can work it out?
Firstly, we need to understand the difference between range and endurance. Range refers to how far a distance we can go for the amount of fuel we have, whereas when we speak of endurance we are talking about how long in time you can keep your aircraft in the air. So hopefully, your response to ATC is how many hours and minutes you can continue making donuts overhead the local reporting point until you run out of fuel.
Understanding what factors affect endurance will help you to re-think your sums, and give you a clear idea of how much time you have left up with the birds in the sky. Let’s have a look…
Weight – If you have a few Sumo wrestlers and their luggage in your aircraft, you must consider that even if your aircraft is balanced and the weight is within limits, it is going to decrease your endurance. Why? The tail plane of the aircraft is going to produce more drag as it works harder to balance the heavier nose-down tendency of the aircraft. More drag means that we are going to need more power from the aircraft engine, and in turn, this means an increased full flow. Especially in jet aircraft, which like to fly as possible, a heavier aircraft will not be able to reach the higher, more fuel efficient altitudes, and therefore will also see an increase in fuel flow.
Configuration – If you are doing racetrack circuits in a holding pattern or doing donuts waiting to enter airspace, then it is important to remember not to deploy flaps willy-nilly or stick out the landing gear too soon. These will significantly reduce your endurance and you could unexpectedly run out of fuel. Put in numbers, flaps and gear extended can increase your fuel flow by up to 150% because of the tremendous drag that they produce.
Altitude – The higher the altitude, the greater the endurance for commercial jets, as jet engines are more efficient in the colder temperatures and higher RPM found near the Tropopause. However, Turbo-Props prefer the middle altitudes, and piston engines are more efficient at mean sea level. So if you are flying a single-engine piston aeroplane, flying at 10 000ft is actually going to decrease your endurance.
Wind – Here is the surprising one, until you think about it – wind does not affect endurance at all! Why? Endurance is about fuel flow to the engine, wind does not affect fuel flow, and therefore has no influence on endurance.
Ultimately, if you are looking for maximum endurance, the lowest fuel consumption is what you are after. If you want to fly for maximum endurance in a piston engine, then the optimal speed is Vmp, which is the speed that represents the minimum power required (check this in the aircraft manual). Simultaneously, flying at the lowest, legally safe altitude that offers adequate terrain clearance will provide you with maximum endurance.
Finally, in case you cannot remember, here is how to calculate endurance:
Endurance = Time (hr)
Fuel flow (kg)
In order to work out fuel flow for a piston engine aircraft:
Fuel flow = Specific Fuel Consumption (Fuel used per unit of power) x Total Power
Image: Bob Adams This file is licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license
Flying for Range
Max Conrad is a legend amongst long distance fliers of and especially in light aircraft. Many of his records for speed and distance still stand. I remember seeing Max Conrad at Placo at Wonderboom Airport prior to his record breaking non-stop flight from Cape Town to St Petersburg, Florida in the USA in a Piper PA-32 Twin Comanche. All seats had been removed and the interior was made into a fuel tank that even precluded any visibility for the pilot out the left side – he even fashioned the pilot seat tank to his body contour. He flew the aircraft from the right- hand seat as that is where a PA-32’s entry door is located. Several hours after his first departure towards St Petersburg, the right-hand engine developed an oil leak and Max had to return to Cape Town on one engine in the grossly overloaded aircraft after 13 hours. The oil leak was rectified, and he set out once again and successfully completed the journey of 12,678.83 km (7,878.26 mi) in 56.8 hours. The aircraft was loaded with 632 US gallons more than the normal 100 gallons capacity of fuel, which made the take-off mass more than double the certified maximum.
He told a group of onlookers that his biggest problem on all his record-breaking flights was to keep the engines hot enough to prevent the plugs fouling. I remember reading that this wonderful man had the highest number of flying hours recorded – in excess of 50,000 hours when he died in 1979.
Max Conrad’s determination of the best speed for ENDURANCE was 1.4 x Vs (Stall Speed) and for best speed for RANGE 1.7 x Vs. On a light aircraft not loaded beyond its maximum certified mass, these speeds shouldn’t range by more than 5-8 knots over the permissible mass range of the aircraft. A simple graph depicting weight and stall speed can be drawn from the Pilots Handbook figures and the FLY FOR RANGE and FLY FOR ENDURANCE speeds can be depicted for easy reference.
Having flown a B747-400 many times between Miami and Cape Town in both directions, I always marvelled at Max Conrad who did it non-stop in a Piper Twin Comanche – let me assure you it is one heck of a long way to fly!
Karl Jensen