Fuel Management
In the last issue of Buzz I wrote about fuel planning and how to ensure that the correct amount of the correct type of fuel was on board the aircraft before departure. Continuing from that, how does one manage this fuel in flight to ensure a constant supply to the engine? It is a sad fact that accidents continue to happen due to engine failure from fuel starvation when there is significant amounts of fuel still in the aircraft tanks. The pilot simply mismanaged the fuel cocks, pumps etc and denied himself the use of the fuel he had on board!
Let’s use a typical low wing single engine machine with fuel tanks in each wing and a fuel cock with no “both” position to illustrate my discussion. Many of us fly high wingers which use gravity to supply fuel to the engine or machines with only one tank in the fuselage and these simplify the problem. Let’s go through a flight starting where we left off last time, in other words we have got the correct fuel in the tanks, done the water check and are now ready to start up.
During the cockpit checks there will be a requirement to test the electrical fuel pump; this is most important as it’s the only time you get to verify the functioning of this back up pump. You will need the fuel cock on first and you should see the fuel pressure rise smartly up to the value stated in your aircraft handbook and stay there. If you don’t get the desired pressure do not even think about flying until you find out why. Yes of course the engine will start and run on the mechanical pump but what if that fails (as they do)? You don’t have gravity working for you in a low wing aircraft and the lack of fuel will cause an immediate and irrecoverable engine failure.
Before you take off you need to verify the fuel flow from both sides of the system; the way to do this is to start and warm up on one tank, then change to the other before taxy and power checks. This gives a few minutes running on each side and proves their functioning, one warning; don’t change just before take off. If the newly selected side is going to fail it may take a minute or so, you don’t want to change tanks, take off then find at 100’ the new side is not feeding fuel after all and the engine was just running on what it had!
After take off the normal convention is to switch the electrical fuel pump off, this is usually done at 1,000’ but ask yourself, why? The pump is selected off to avoid wear and preserve its life; fine but, what if the mechanical pump has failed during the take off and the only thing supplying fuel pressure is the electrical pump? In this case the engine will stop as soon as you switch off the pump; I’d rather not have that happen at 1,000’! I recommend switching off the electric pump at top of climb or at least leaving it until 2,000’ agl.
So, now we are in the cruise, all we need to do now is keep the fuel in balance and navigate, pretty straightforward! Well, remember what I said last time; I don’t trust fuel gauges in light aeroplanes. So, how do we keep track of the fuel used, the fuel remaining and the balance and how do we crosscheck our progress against the plan? We need to know what we should have left at any time so that we can track that against what the gauges say we actually have left and, to do this we need to use fuel in a methodical way.
Try this; when the minute hand of your watch is in the left side of the clock face, use the left tank, in the first half of the hour use the right tank. By doing this you will always know which tank you should be on and therefore whether you have forgotten to change or not. More important, if you had a note of the contents of each side before take off (from dipping the tanks because you don’t trust gauges) you now have the information you need to work out what the expected contents remaining in each side should be at any time and you can therefore verify what the gauges are telling you and crosscheck it against the plan. I construct a table on my knee-board like this;
Here’s how it works. I took off at 10:42 with 30 Litres a side, I was using the left tank as it’s in the second half of the hour and my fuel consumption in the cruise is 20 L/hr. At 11:00 I changed to the right tank and noted the expected fuel figures on the table. I can now check this against the gauges confirming that the fuel consumption is going according to plan and also that the reading on the gauges is close to what I expected.
Let’s say that at 11:05 I did a FREDA check and found that the left tank was selected, this means I forgot to change at 11:00. As you can see, I now have a framework which tells me which tank I should be using and when I should change so that I will know if I’ve forgotten. While we’re on FREDA checks, you should do these at top of climb, every 10 minutes in the cruise and at top of descent. When ever you change tanks you must switch on the booster pump, change over, check the fuel pressure is maintained then, after 30 seconds or so switch off the pump and monitor the fuel pressure again for any drop. If there is a problem with fuel flow from the tank you have just selected the first sign will be a falling fuel pressure just before the engine stops.
At 11:30 I changed back to the left tank and noted 24L remaining in the left and now 20L in the right. Let’s imagine that, at 11:50 I suspected a fuel leak. I can simply look at the table and instantly I know I should have consumed about 8L from the left tank so I should have 16L in the left side and 20L in the right, if the right gauge shows 10L I know I have a problem (either leak or dodgy gauge).
Lastly, most checklists call for the booster pump to be selected on during the downwind checks. Remember again that the purpose of this pump is to provide back up fuel pressure in the event of a failure of the main mechanical pump. I personally want the protection of back up fuel pressure before I get to 1,000’ on the downwind leg so I select it and leave it on during my FREDA check just prior to top of descent.
There we have it then, a simple framework which allows you to ensure that fuel is available from both sides before you take off, reminds you when to change and gives you the information you need to compare the expected fuel on board against the indicated fuel and so detect any problems before they become serious.
One final note, although most low wing aircraft need the fuel pump on for take off and landing some do not and putting it on can cause over-pressurisation of the system (Bonanza for example) so, read the aircraft manual to make sure you understand the system and operate it as the designer intended!
By Dave Mackay
