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