Jim Davis – Fuel Systems – why so complicated?

You know those aggranoying people who say, “Oh I wouldn’t fly in one of those little things – you can’t just pull over and fix it if something goes wrong”?

What makes this particularly irritating is that the bastards are right.

Pupes, pilots and instructors lend me your ears. By far the most common reason for aeroplanes to ‘break down’ is because the pilots didn’t understand the fuel system.

There are at least twenty things that you have to know about your aircraft’s plumbing if you plan on a long and healthy future in aviation. I’ll tell you about them in a moment.

Now, let’s see how that compares with a car which has just three that are nice to know:

• Which side the filler is on
• How to open it while sitting in the car
• It’s still got some gas after it shows ‘E’

You don’t even need to know whether it’s petrol or diesel – the refuelling guy will tell you if you are interested.

And if you don’t get these things right in your car, the worst that can happen is that you have an unplanned stop while you phone the AA.

You really don’t have to understand cars, or your watch, because they just keep working despite your ignorance.

Okay cowboy – we are not talking about your boet’s E-type with triple SUs, and we won’t mention Land Rovers – we are thinking of normal little boxy cars that all look the same and don’t break down.

But aeroplanes are not that easy – and they DO give trouble, and phoning the AA won’t increase your life expectancy. So let’s look at those 20 things you need to understand in order to avoid a worrying silence ahead of the firewall.  

Fuel Gauge (#1)

These are NEVER to be trusted. The problem is that wing tanks are often of such a shape that it’s not possible to make gauges that tell the truth under all circumstances. We discussed this at length last time I saw you.

Fuel Pressure Gauge (#2)

This measures the combined pressure of the mechanical pump and the electric pump which both feed the carburetor. The normal pressure is somewhere between about two and seven psi.

Have a look at the diagram and try to figure what would happen to the fuel pressure if you accidentally left the fuel low-point drain on (#15). No, I’m not going to tell you – this is all part of understanding and being involved.

A fuel pump is needed on low-wing aircraft to raise the fuel from the tank to the carburettor. Many small, high-wing aircraft don’t even have a pump – they rely on gravity to do this job.

Don’t confuse fuel pressure with fuel flow. We are keeping this simple and just dealing with carburettor engines – not fuel-injected ones that have fuel flow gauges.

However, while we are on the subject, if you are flying a fuel-injected aircraft, fuel flow is absolutely critical. I’ll tell you about it in a minute.

Ball Slip-Indicator (#3)

Thinks, maybe the vicar has become deranged, I thought we were discussing fuel. Fear not, dear parishioner, we are all singing from the same hymnbook. Those who were paying attention to previous sermons will remember that the fuel goes where the ball goes. So, if you are sideslipping to the left and you are selected to the left tank, it’s possible the engine will starve.

And remember that Madame Beech warns that a rolling takeoff can cause the engine to die. She means if you do a cowboy turn onto the runway – say to the left – then the fuel may slosh away from the outlet on the right tank and cause you some despondency during takeoff.

Electric Fuel Pump Switch (#4)

Oh boy, this little goody can kill you in seconds if you get it wrong on takeoff or on aircraft propelled by Conti fuel-injection engines.

If there is one switch in the whole aeroplane that should be labelled FFS THINK, it’s this one.

This isn’t the place to go into details of how the fuel-injection system on a six-cylinder Continental engine works, but briefly this is what happens. The electric fuel pump switch has three positions: OFF, Hi and Lo. The Hi position is ONLY used to purge the system of vapour locks. If you use it at any other time the engine will stop. It must NEVER be used on takeoff.

Don’t take my word for it – see what the POH says for your particular aircraft.

With the six-cylinder, fuel-injected Conti, the pump must be on, but only in the Lo position.

I keep seeing accident reports, often fatal ones, where a Bonnie or a C210 or a Seneca has crashed and burned on takeoff because the pump was selected to the Hi position.

You have been warned.

While we are on the subject, it’s critical with a fuel-injected engine that you keep a sharp eye on any fluctuations in fuel flow during takeoff. The slightest flicker or wandering of the needle calls for an immediate abort. Don’t assume it’s just an air bubble and wait to see what happens – take it as a warning that all is not well.

Finally, a good pilot keeps his finger on the fuel pump switch and watches the pressure, or flow, for a few seconds after switching it off.

Fuel Level Step (#5)

Piper has got this dead right – it’s a really useful bit of clever design. On the Cherokee range, the main tanks hold 25 US gallons each. If you are on level ground and the fuel is at the step, then you have got 17 gallons which means that the tank is 50 lbs lighter than when full.

To keep it nice and easy – if you tell the fuel guy to fill it to the step, then you know you can carry an extra 100 lbs in the cabin.

And if you want your 140 or 180 Cherokee to be in the ‘utility’ category for training, then the rule of thumb is no rear seat pax, no luggage and the fuel at or below the step. Easy peasy Cherokeesy.

Breather (Cessna only) (#6)

In contrast, this little pipe joining the two tanks on the 100 and 200 series Cessnas is about the worst piece of design I have ever seen. It has been directly responsible for hundreds, perhaps even thousands, of these aircraft running out of fuel.

For reasons no one seems to understand, Clive Cessna decided he only wanted one breather – and he put it under the left wing of these aircraft and connected it to the left tank. Then he ran a troublesome pipe across to the right tank, so it too could breathe.

The purpose of a breather is to allow air into the tank to replace the fuel as it’s used. If the breather gets blocked say by ice, or by a bug making its home in there – and this can happen in a few hours – then one of two things will happen during the flight. Either the fuel will stop flowing after a while, or the rubber fuel cell – if the tank has one – will collapse and scrunch up into a little ball which will hold about two gallons next time you refuel it.

Mr Cessna’s stupid little pipe not only allows air to flow from one tank to the other – but obviously fuel as well. This has all sorts of consequences, but the main one is that if you get say three hours out of the left tank then you would expect the same out of the right tank. It won’t happen – you have probably only got an hour-and-a-half in the right tank because the fuel cross-feeds as you fly.

This is an extremely dangerous, and surprisingly poorly understood, characteristic of the stupid system.

Also, if you fill both tanks in the evening ready for an early departure the next day, they will only stay full as long as you are parked on level ground – and I mean absolutely level. If it’s slightly left wing down – even a soft tyre can cause that – then the right tank will cross-feed into the left one and this will overflow through the breather and disappear into the sand or grass without a trace. And the longer it does this, the lighter the right wing becomes and the more the aircraft will lean to the left.

Fuel Cap (Piper with breather) (#7)

This is a surprisingly complex bit of engineering. Think what is has to do. It must act as a breather to let air in – but not water, despite being left outside in torrential storms or even flown through TSs. It also has to prevent fuel sloshing out.

It also needs to be aerodynamically unobtrusive. Ideally, it’s sunk into the top surface of the wing and has a nice smooth cover over it. But hang on, if water gets past the cover then the cap will soon be submerged by the water filling up in the recess. That’s no good, so they put a nice big drain pipe out of the cavity – and we have to make sure that bugs don’t build their condos in there.

Clyde Cessna and Victor Roos bumbled around for 50 years trying to design fuel caps that keep the water out, let the air in, and keep the fuel in. They kept producing modified caps that didn’t really work. Eventually, they decided to hell with aerodynamics, and built a big solid thing that looked like it came from the radiator of a 1932 Fordson tractor. This seemed to work, so they painted it red to let you know you have got the right one at last.

Sorry to harp on about Piper, but the Cherokee caps have never given anyone an ounce of trouble and they cause very little inconvenience to the airflow. You need to look very, very carefully to see how they work. On the photo, the bottom arrow points at a small square section groove that runs above the main gasket and vents into a cavity behind the circular red valve in the centre. This is made from paper-thin red rubber which lifts slightly to allow air in but easily withstands the pressure of fuel sloshing against it. It never lets water in.

float for fuel gauge (#8)

These seldom give trouble – and they also seldom give accurate readings. We have discussed this endlessly. The only thing you really need to know is that all fuel gauges are not to be trusted with your life.

fuel tank breather (#9)

Enough already – we have discussed breathers ad nauseum. On your Cessna preflight you will find the breather pipe sticking out below the left wing – just behind the strut. It is bent to face forward into the airflow. And the Cherokees have one under each wing near the tank – it sticks down about 15mm and it is cut off at an angle.

It’s also a terrible piece of design because it’s small, and almost impossible to inspect for bugs.

I wouldn’t be surprised to find that many of the Cherokee ones are blocked. Fortunately, the excellent fuel cap breathers probably save the day.

outlet + coarse filter (#10)

The position of the outlet determines how much unusable fuel there is in the tank. Obviously any fuel below the level of the outlet is unusable. Why should we want unusable fuel? Actually we don’t. If we could be sure that all the stuff at the bottom of the tank was nice clean fuel, then we would really like to use it.

The trouble is that dirt and water are likely to go to the bottom, and we don’t want them getting into the system.

The fuel drain is right at the very bottom of the lowest point, so in theory we should be able to get rid of any crud and water by draining it there. But that’s not always the case – we’ll discuss that next.

The coarse filter prevents any large objects – like coins or nuts and bolts, which you might accidentally drop into the tank – from blocking the outlet pipe.

tank low-point drain (#11)

This should be easy – you drain a bit into your little transparent goodie, make sure it has no water or sediment in the bottom, check it’s the right colour for avgas (light blue), confirm it smells like avgas, and dispose of the sample. You do this either by pouring it back in the tank, or chucking it on the concrete where it will evaporate – but not on the tar, because it eats tar.

But what if the tank doesn’t really have a low point because the aircraft is parked one wing low? Or because it’s parked nose down, or has a soft nose oleo? Or because the floor of the tank has slight ripples in it – particularly bladder tanks?

All these are common problems – particularly on high-wing aircraft with little dihedral.

On the 100 and 200 series Cessnas, the original one drain point per tank was found to be insufficient. The drains were positioned at the rear end of the inboard sides of the tanks. This was because these aircraft have very slight dihedral and normally sit slightly nose up. So everything is fine as long as you do your preflight on perfectly level ground – and you hope the tank has no wrinkles in the bottom which could contain little dams of water.

It took a few years for Clyde and his mates to realise there was a problem, but when they did, they rushed forward with a mod which pretty much sorted it out. They got everyone to fit two – or sometimes three – more drain points per tank. So some Cessnas have eight drain points, which sorts out the pitch problem.

Jawellnofine. But that still left the wing-down and wrinkle problems. At this stage, Clyde Cessna lost it.  He hurled his slide rule to the floor in desperation, and sent us all a letter saying, just make sure the wings are level and rock the aircraft from side to side to clear little dams of water. And, if necessary, get someone to hold the tail down so the water goes to the drains at the back.

If you didn’t get his letter, don’t take it personally, but just do as he says.

Now it’s Pug Piper’s turn to catch it in the neck. Briefly, he caused me to lose both engines in an Aztec because, when I thought I was draining from the low points on the tanks, I was actually draining the fuel in the pipes that travel from the low points to the remote drains below the engines. There were gallons of water in all the tanks, it’s just that I wasn’t draining the tanks – I was simply draining fuel from the pipes.

Even worse is Piper’s stupid system that lets you drain all the tanks from within the cockpit. It’s impossible to see the colour properly through a yellowing plastic pipe. Equally, you can’t smell the fuel, or spot water or sediment. This is on the Comanches and Saratoga series.

Just past halfway on our 20 point fuel system. The rest at evensong.