Jim Davis
Yep, we are still thinking about engine food. When we started down this road, I had no idea it was going to take five times longer than any other part of the stuff instructors should bestow upon their congregations.
Let’s start with the story behind Joe’s cartoon – it has two serious morals. First your pupes need to know that a good earth is crucial before they touch the nozzle to the tank. The second is that if you happen to have set fire to a drum of fuel, you can either wave your arms around and run for the hills, or you can just put your hand, or a lappie, over the hole, or put the lid on.
I always imagined it would explode until Peter’s unintentional demo showed me how wrong I was.
Anyhow, here’s what happened.
I was between marriages and had a girlfriend, the fair Yvonne, in Salisbury, which now identifies as Harare.
My mate and student, Arthur Westworth, agreed that a dual cross-country to Salisbury and Vic Falls sounded like a brilliant way of spending a few days over the Christmas hols. We took my almost new 180 Cherokee ZS-ELK and collected Arthur’s girlfriend, Hazel, in Durban. We cleared customs at Virginia and set sail on the long leg that took us past the magnificent Drakensburg mountains, across the Limpopo and into Beit Bridge to clear customs and immigration into Rhodesia.
In those days you didn’t need to tell the guardians of the border that you were planning to drop anchor – you just beat up their shed a couple of times, and landed at Peter Dahl’s strip, just north of the river.
The officials, in their white uniforms, enjoyed the diversion from form stamping and paper signing in the office. They would spring into a Series 1 Landy, drive out to the airfield, and take their time anointing our paperwork. It was a peaceful and relaxed procedure – they were never in a hurry to get back to headquarters.
Peter Dahl, owner of Staggerwing ZS-PWD, also heard us. He flang an armful of cold Cokes into his WW2 Chev truck and clattered out to the airfield.
Peter was always fascinated by machinery and gadgets and loved fiddling with such things, He was usually very good at it, however, I viewed his latest refuelling contraption with some suspicion. He had rigged up an electric pump to transfer the fuel from the drum to the aircraft.
I didn’t like it and tried to point out that we could happily syphon the fuel since the drums were nice and high, but Peter was determined to demonstrate his apparatus. It sprouted red and black leads, that Peter clipped to the Chev’s battery.
At that moment, something else took my attention, and I only looked back when I heard Peter bellow “Oh f##k” from the back of his truck. My worst fears had come true. A surprisingly small fire was coming from the open mouth of the drum, and a considerably larger one enveloped the pump which I observed in mid-flight as he hurled it away.
Fortunately, we hadn’t put the nozzle in the aircraft’s tank when the conflagration broke out. So the pump and rubber pipe burned happily in the hot Messina sand, while Peter put his big leathery hand over the flaming hole in the drum.
All too soon the fun was over. Peter was unharmed and unconcerned by the near disaster. We all drank our Cokes while he muttered darkly about earthing.
The rest of the Christmas holiday was as good as it gets – 30 hours with pretty girls, blue skies and the wonders of Africa to explore.
Some may remember that Arthur was a big noise in ballooning circles and later became the boss of the Aero Club of SA. He was also a hell of a good guy. Sadly, in February 2005, Arthur was shot and killed by intruders in his home in Kyalami, while Hazel, who was then his wife was forced to look on.
***
Okay you lot, enough chatter – time to turn our safety focus on understanding the rest of the fuel system:
16. Electric fuel pump.
If you have a look at the beautiful diagram you will see that the electric and mechanical fuel pumps are connected in parallel. That means that if one fails or gets blocked the other can do the job just fine, thank you.
So the electric fuel pump is little more than a backup for the mechanical one that automatically pumps fuel as long as the engine is turning.
Many high-wing aircraft don’t even have an electric pump – they rely on comrade Newton’s gravity to shove the fuel from the tanks to the engine.
But of course low-wingers don’t have that going for them. This means that they need an electric pump to get the fuel to the engine before it can start.
You should test the electric pump before start to make sure it brings the pressure into the green. And once the engine is running you need to test the mechanical pump by turning the electric pump off and noting that the pressure remains in the green.
You should also use the electric pump during takeoff, landing and low flying, and any other time when a mechanical pump failure would be troublesome.
The electric pump will speed up an engine restart in flight if you run a tank dry. It normally takes between 10 and twenty seconds for the engine to come back to life – but if feels like about a week.
Fuel injection engines may call for you to study the POH on how to use the electric pump during hot and cold starts. Be extremely careful with Continental fuel injected engines that have Hi and Lo positions for the electric pumps. Get this wrong on takeoff and you are likely to lose power and crash. I have reported on a number of fatal accidents with C210s and Bonanzas where the pilots used the Hi position during takeoff. Study the POH – you have been warned.
17. The mechanical fuel pump
The mechanical fuel pump moves the fuel to the carburettor, or fuel injection system, so long as it’s not broken and the engine is turning.
This statement is usually true – but not always.
Let’s have a pop-quiz to see if you have been paying attention to the diagram and the previous 500 pages on the fuel system. Ready?
List everything you can think of that could cause the engine to stop even if both pumps are in perfect working order.
Give it some thought before you look at my 20 point list, it will help to have the diagram in front of you. I’ll get you started by telling you the pipe to the pumps is either empty or it contains water, because of something you have done, or failed to do:
• The selected tank is empty because you ran it dry.
• You didn’t allow for contingencies like headwinds, incorrect leaning, getting lost and bad weather diversions.
• The fuel was stolen overnight and you didn’t do a thorough preflight.
• You trusted the fuel gauges.
• You didn’t use a dip stick.
• One of the tanks is leaking.
• A fuel drain was leaking or left open.
• The fuel was sucked out because a cap was loose, or left off.
• The ball is out of the middle, causing uneven left/right consumption figures.
• A sideslip or rolling takeoff caused unporting.
• The tank filter is blocked. Not your fault.
• The low-point filter bowl is blocked. Not your fault.
• The low point filter bowl contains water or gunk. Your fault – you should have checked.
• The tank breather is blocked.
• The cap breather is blocked.
• You selected an empty tank.
• The selector is faulty.
• The selector’s markings are missing or incorrect.
• The tanks contain water or something else.
• You are using the wrong fuel.
Now, once the fuel arrives at the engine, we have three controls – the throttle, the mixture and the carb-heat – to handle its final journey before it gets burned.
A quick reminder that the carburettor is a device, about the size of half a loaf of bread, and its job is to vaporise the fuel (mix it with air) in the ratio of about 1:15. That’s one part of fuel to fifteen parts of air, by weight.
The Throttle
This controls a butterfly valve which allows more or less of the fuel/air mixture to reach the cylinders. So it controls the power.
On most aircraft the throttle also operates a tiny little pump that squirts in a bit of extra fuel as you open the throttle. This is called an accelerator pump and its job is to make sure that ham-handed pilots who bang the throttle open suddenly, don’t cause the mixture to temporarily become so lean that the engine splutters, or even dies during throttle opening.
But you wouldn’t do that would you? Or allow your pupils to, because we all know that smooth engine handling is the hallmark of a good pilot.
Mixture
Perhaps the mixture control is the most abused engine control in the cockpit. Those pupils and pilots who have no mechanical aptitude will never really understand it. But everyone can learn general rules that will make sure they operate it safely.
First, what does a mixture control do? And why do we need it?
It allows us to adjust the fuel/air ratio of the mixture entering the cylinders. Remember we said earlier that this is ideally around 1:15. The reason we need to fiddle with it is because as we climb to higher altitudes the air gets ‘thinner’. And this messes with the ratio, so the 15 parts of air may become say 12 which would mean that the engine would be eating a mixture of 1:12. The same amount of fuel but less air. In other words the mixture would be too rich.
A rich mixture means the engine is less efficient – it produces less power, and it is prone to foul the spark plugs, and possibly make the engine run too cold.
On the other hand, if we make the mixture too lean – meaning insufficient fuel for the amount of air, the engine will lose power dramatically and will tend to overheat to a dangerous level, causing burned valves and detonation. Detonation is a condition in the cylinders where the fuel explodes violently instead of burning in the normal way. It can physically damage the pistons and may cause them to break, with dramatic results.
Got that?
• Too rich – a loss of power.
• Too lean – a major loss of power and possible engine damage.
So here are some general rules – which are just that – they are no substitute for the POH.
1. On the ground, while idling or taxying, lean out aggressively to avoid plug fouling.
2. For max power during takeoff at altitude, lean for max revs but be on the rich side.
3. In the climb don’t lean out below 5000.
4. In the cruise only lean out if you are using 75% power or less.
5. During your descent the mixture becomes leaner and leaner as you enter denser air. Ideally you should richen it a bit every thousand feet or so, but you are likely to forget, which could mean that, when you need power, you will have none because the mixture has sneakily become too lean. If you think this can happen, then richen the mixture at the top of decent, when you reduce power, and then you have one less thing to think about.
6. If you change the power setting, even a little bit – you need to lean out again.
7. You may hear of all sorts of fancy ways of leaning out for best power or best economy or lean-of-peak. Generally they only apply to fuel injected engines fitted with special EGTs – one for each cylinder. For us ordinary people the rule is to lean out slowly until the engine just starts to run a bit rough – then richen for smooth running and then a minute amount more to be on the safe side.
8. I don’t trust an EGT because you never know whether it’s fitted to a rich or a lean cylinder. Yep, the mixture does vary slightly between cylinders.
9. When you have the mixture fully rich, at sea level, the engine is slightly richer than ideal, typically about 1:12. This reduces the possibility of pre-ignition or detonation, and helps with cylinder cooling
10. It’s normal to stop the engine by bringing the mixture fully aft into the ‘idle-cutoff’ position. This prevents the engine from erratic behaviour during shutdown. It also prevents fuel from diluting the oil on the cylinder walls.
11. The POH is always king.
Carb-heat
Now we have yet another thoroughly misunderstood control.
What is it, why do we need it, and how does it work? Instructors, remember the What Why How rule for training?
What is it? It’s a pilot-controlled device for diverting hot air into the mouth of the carburettor.
Why would you need to do this? Because, under certain conditions, ice can form inside the carb and this will reduce or even cut off the airflow causing a reduction in power or a complete loss of power.
How do you use it? Okay here are some more general rules.
1. If you have a fuel injected engine there’s no carburettor to ice up, so there’s no carb-heat. Lucky you.
2. Never use carb-heat with full power. (a) you won’t get full power because you are using hot – less dense – air. And (b) the less dense air makes the mixture too rich. And (c) you can damage the engine because if may cause detonation.
3. Always clear any possibility of ice before leaning out. If you lean the mixture while there is ice in the carburettor then the mixture will be too lean when the ice melts.
4. Generally the POHs for small Cessnas call for carb-heat whenever you throttle fully back as when gliding or practicing stalls and spins.
5. Generally the small Pipers POH’s will tell you to only use carb-heat if you have an indication of icing.
6. Almost all small aircraft engines will gather ice when taxying over wet grass – particularly on cool mornings.
7. If, when you throttle fully back to stop at the holding point, the engine tries to die – that’s a sure sign you have picked up ice while taxying.
8. Always clear any chance of carb-icing before you do your engine runup and mag-check.
9. The temperature in the carburettor can drop by as much as 38°C (100°F).
10. Beware of high humidity, particularly when cruising just below cloud.
11. When you apply carb-heat you must expect the revs to drop about 150 RPM.
12. If ice is present the revs will drop more and if there is a lot of ice you may get a major drop in revs, perhaps 500 or more, and the engine will run extremely rough. Don’t get a fright and slam the carb-heat off. That has caused many 150s and 152s to do unnecessary forced landings. Be brave – keep the carb-heat applied and life will return to normal.
13. RTFM (Read The Factory Manual).
Final tip:
If the engine stops, without expensive noises, it’s almost certainly a fuel problem:
• Close the throttle – you don’t want the engine surging when the new fuel comes through.
• Change tanks
• Switch the pump on
• Richen the mixture