The
Essence of Life
(or; Fuel, how much is enough ?)
Most of us fly single engine aeroplanes (and some in helicopters) and we consequently rely rather heavily upon our engines. There are three things which an engine needs to receive in precise quantities and at just the right time to continue operating for us; these are, of course fuel, air and sparks. Some might say that I should include oil in this list but I’ve left it out because an engine will continue to run for a while (sometimes quite a long while) without its oil supply and, when is eventually does succumb it will fail catastrophically by destroying its bearings and other internal components. The supply of fuel, air and sparks to an engine is, to at least some extent, controlled by the pilot and, if for some reason the supply is interrupted and the engine stops it will not suffer any serious mechanical damage. When the missing ingredient is restored the engine will quite happily resume its operation with few significant ill effects.
This is the first of a series of short articles and it will deal with fuel, later articles will deal with the air and sparks and all will be written from a practical viewpoint to suggest some things which we, as pilots should do to avoid joining the long list of accident statistics. It is quite possible to fill a book (and there are many good ones available) on each of these subjects so I’m going to concentrate here on practical steps a pilot can take to keep out of trouble.
So, here is a simple question: how much fuel do you need? As with so many things, the answer is not as simple as the question and the question itself begs further questions. Let’s take a practical example; we are preparing for a VFR flight in a single engine aeroplane from Yeovilton to Duxford. We first need to check that the weather is good and there are no relevant NOTAMS as either of these things can dramatically affect our route and hence our fuel plan. Once we have these answers we can begin our fuel planning.
The main thing we need is to calculate fuel for start-up, taxi, take-off, climb, cruise, descent, landing and taxi in; in other words the expected fuel burn. This is the simple bit but note that we must have calculated the expected flight time allowing for wind and the planned cruising levels in order for the climb and cruise fuel consumption to be accurate. Of course we don’t just go off with this amount of fuel in the aeroplane, there are extras which have to be added and this is where we get to the crux of the matter, “how much is enough?”
Here is my answer. First, find out from the aircraft manual if there is any unusable fuel and whether or not it is shown on the gauges, if it is you need to start by adding this in to the calculation. Take the expected fuel burn, add 10% to allow for such things as unexpected headwinds weather avoidance en-route etc (in a slow aircraft where winds can have a more significant effect you might want to allow 15% or even 20%). You should always plan to land with at least 30 minutes of usable fuel remaining, calculate this and add it to the pot. This is the absolute minimum fuel with which I would depart for the above trip but note that there is very little margin to cater for the unexpected and I would only use this fuel plan if I was absolutely certain that I would have good weather, that the destination airfield was open etc. What if someone ahead of me in the circuit crashed and Duxford closed unexpectedly 5 minutes before my arrival?
In most cases it is wise to plan fuel for a diversion to another suitable airfield (don’t forget to increase this figure by 10% to 20% too) and often (e.g. if flying to a busy airport like Yeovilton) it might also be wise to allow extra fuel for holding at the destination before landing, I recommend an extra 30 minutes. We have now reached what I consider to be a normal fuel state for departure on a straightforward VFR cross country flight.
If the flight is IFR two things change. An alternate is no longer optional, the plan must include fuel to reach an alternate airport with suitable weather and similarly the holding fuel is no longer optional, you must include fuel to hold for 45 minutes.
On top of this you can add further extra amounts as you see fit but beware of exceeding any weight and balance limitations.
Here is a sample fuel calculation for the above trip in my Cessna 120;
Item Fuel required
Unusable fuel
Expected burn
10% extra
Minimum landing fuel (30 min)
Minimum departure fuel
Diversion to Cambridge (inc 10%)
VFR Holding (30 min)
Planned departure fuel
Extra if required
Actual departure fuel
10L
33L
4L
10L
57L
9L
10L
76L
??
??
Remember that if the trip is IFR both the diversion fuel and 45 minutes of holding fuel are mandatory and must therefore be included in the red “Minimum departure fuel” figure. Note that you should think very carefully before departing with less than the amber figure and you should never depart with less than the red figure. If you find yourself at an airport with no fuel available and less than the red figure in your tanks you must think again, either wait till fuel is available, or change to an en-route destination which is closer (re-plan the figures) and refuel there before continuing on to the ultimate destination. Do not even think of departing for the original destination and hoping things will work out along the way, they will not and you will run out of fuel and crash!
Having gone through this planning process all your efforts will be wasted if you don’t get the correct amount of the correct type of fuel into the tanks! Obvious I know but worth emphasising nevertheless. There are ever more light aircraft around now with diesel engines, they run on Jet A1 fuel but they will not run on Avgas, nor will spark ignition engines run on Jet A1. You must check that the correct fuel is being supplied for your aircraft. The use of Mogas is a big subject and outside the scope of this article, perhaps our engineer might write something on that subject! In any case, you must see that the correct type of fuel is being loaded aboard your aircraft.
As well as checking that the correct type of fuel is being used, you need to see that the correct quantity is aboard. The fuel gauges in light aircraft are rubbish! No exceptions, I don’t trust them! I will not take off until I have physically verified the contents of the fuel tanks, either visually on a low wing aircraft or with a calibrated dipstick on a high winger (you will probably need a stepladder or a chair to stand on, if there’s not one available find something suitable, get someone to give you a leg up, do anything you need to but do not omit this crucial check). Do not use any old stick or a screwdriver etc as a dipstick, such things could introduce dirt into the tank or you might even drop them inside (it has been done!). Make or buy a proper dipstick, calibrate it by filling the tank from empty in 5 Litre steps right up to full marking the stick as you go (make sure that you are parked on level ground) then keep it in the aircraft for future use. I have used Litres in this article but many aircraft use US or Imperial gallons, most fuel suppliers use Litres these days so you must be able to do the conversions. Most flight guides have some form of ready reckoner and you could make up a table to keep in your flight bag or in the aircraft to simplify things. In any case, if you always dip your tanks you will not fall into the trap of thinking you have taken on 20 gal when you have in fact only taken on 20 Litres!
Having got the correct quantity of fuel in the tanks you need to see that it does not contain what it shouldn’t, i.e. water. You must do a water check on all tanks before the first flight of the day, after every re-fuel or if the aircraft has stood outside in any significant rain.
Now we have the right amount of fuel in the tanks we can set off on the flight. The next in this series will cover getting the fuel from the tanks to the engine and what the pilot can do to manage the fuel system in flight. In the meantime you could cut and paste the fuel planning table from here, print yourself a copy for your flight bag, read your aircraft manual to check if there is unusable fuel, make up a conversion table and remind yourself of the fuel consumption figures for your aircraft.
Happy planning!
By Dave Mackay
