Thursday 05 December 2013 06:56 UTC
Discuss the problems and solutions to all of the situations that Pilot X finds himself in.
Pilot X figures that a half-full tank will still leave him plenty in reserve − but are there any factors he hasn’t taken into account? By Adrian Bleese
Pilot X could remember with pride all of his aviation firsts: his first flight, his first solo, his first cross-country, his first trip with a passenger… yet here he was on the day of his very first solo glider land-away and he wasn’t feeling proud at all. He had to admit that this was probably due to the fact that it had begun as a local flight in a Cessna 150 and the gliding element was entirely unintentional.
He ran through the checks to determine the reason for the unrequested silence: primer, mags, throttle, mixture and somehow he found that he also had time to reflect on the events which had brought him here…
It had all started in the pub, the night before, when he’d agreed to take his friend, Ed, for a flight, meeting him at a farm strip close to Ed’s home. Pilot X had phoned the flying club early on the Saturday morning to arrange not only the hire of an aeroplane but of one with significantly less than full tanks. He’d had to do some quick maths because, while Pilot X was obviously a CAA standard 90kg, Ed was 16 stone in just his pants and Pilot X had no wish to see him in his pants. The club’s Cessna 150L weighed 1,127lb with just its oil; so, with both of them on board, Pilot X made a quick mental calculation that he could afford to have just 50lb of fuel in the tanks for their trip.
A check of his PPL notes showed that a gallon of fuel weighs 7.2lb but that was as good as seven gallons and he’d always planned for five per hour during his training. That meant that he could afford to fly Ed for 45 minutes and still have two or three gallons, giving him half-an-hour’s reserve, just in case. As they were meeting at a farm strip which was 20 minutes flying time away, he could also afford to put in another gallon-and-a-half which he should have burned off by the time he got there. That was his plan, then: eight-and-a-half gallons would give him 20 minutes to the strip and 45 with Ed, leaving a more than adequate reserve. He could then top up before flying home again; the maths worked.
Just enough fuel
It was mid-afternoon before he got to the airfield. He’d not only left plenty of time for last night’s couple of pints to wear off but had had to placate his girlfriend about disappearing again. It took a promise of dinner and a movie to get him out of trouble, meaning that he had to be back home by 6.30 or face the prospect of finding somewhere new to live. He found the little Cessna with quarter tanks, which was ideal. His walkround confirmed that three-quarters of the dipstick was dry; with a capacity of 26 gallons it must currently have six and a smidge, so Pilot X needed just two-and-a-half.
He taxied to the pumps and shut down. He’d always filled aeroplanes to the top before and hadn’t realised that the pump was in litres, but someone had very helpfully written the conversion factor on the cover of the refuelling log; one gallon equals 3.785 litres. A scribbled calculation later and, adding half-a-gallon for safety, he was pumping just under eleven-and-a-half litres in.
As he taxied to the holding point, the gauges still showed less than a quarter full,
but then everybody knew how unreliable they were, he must now have nine gallons, more or less. It was a beautiful early spring day, a powder-blue sky and some high up, puffy cirrocumulus. It was one also of the first times this year that the sun warmed the side of his face as he flew. Twenty minutes later he was rounding out at the gorgeous, wide green farm strip to pick up Ed.
He could tell that his friend was a little nervous and so decided not to mention the
fuel calculations and just get on with flying him over his house and bringing him back to earth without too much of a bump. Ed said that he enjoyed himself, but his colour suggested that half-an-hour was enough and Pilot X rejoined the circuit.
Ed’s colour returned as Pilot X’s drained − there was no fuel, apparently there never was, people sorted themselves out. Well, usually they did. It was after five in the afternoon and finding the owner of a high-wing aeroplane with a syphoning tube and a jerrycan was a non-starter.
OK, time to think seriously. He had started with six gallons and added three; at five gallons per hour that was enough for almost an hour-and-fifty-minutes and he’d flown less than an hour. Even being pessimistic, he needed just two gallons to get him back to the airfield and he had at least that. It was now nearly five-thirty but he might still be home in time to save his relationship, if he got straight off.
Ten minutes later and almost in sight of home he heard the first cough from the engine, then another, which he felt…
Carb heat didn’t seem to help. The third cough was the last, then silence. He stared in disbelief at the slowing propeller and time stretched to allow him to watch in wonder as it windmilled uselessly. The primer is locked, mags are on both, throttle is open, carb heat is on hot and the fuel… ah, the fuel.
The bubble of extended time seemed to focus the world outside. He now remembered that an imperial gallon weighed 7.2lb but a US gallon weighed just six, because it was smaller and, while the C150 had two 13usg tanks, there were one-and-a-half somewhere, which were unusable. He didn’t need to do any more maths or any more checks… he picked his field.
1 What factors were detrimental to Pilot X’s decision making?
2 At what point should he have admitted that the maths just didn’t work?
3 What other factors added to his confusion?
Not withstanding initial mistaken maths, he cut it too fine none the less.
- dipstick wasn't calibrated: with less than half empty tanks it is easy to misread 5-6 litresmore or less, especially if wings are not parellel
- he didn't calculate unusable fuel
- he didn't verify available fuel at destination
- he shouldn't have taken off with less than an hour total reserve considering 3 flights: it is easy to add 10 minutes to each flight due to go around, holding, front wind, ecc
- he was under pressure to get home
Happy only when flying
Yes well, 7gph might have been better, and a one hour reserve might have been better, thus with 7 gallons the allowed flight time is zero. Quite apart from not taking account of unusable fuel (as others have said) and fuel needed for take-off and climb.
Of course he could have just filled up the tanks and gone flying anyway (guessing that an overweight 150 might behave much like an overweight 152 - does it??) ... which might have worked fine on the 2km tarmac runway at the flying club, but I see that the question sneakily mentions a "farm strip" which rules out that solution
The real fix would have been to rent a 172 instead.
- Allowing himself to be put under time pressures and get-homeitis
- not allowing for unusable fuel
- 5 imp gals per hour is marginal, I'd have planned with a higher burn than that, and allowed for some diversion fuel on top of reserves. 50lb of fuel is just not enough - he should have binned the prospect at that point.
- Assuming dipsticks are linear takes no account of any dihedral effect or irregularity in shape of tanks. Did he dip both sides?
- The 3.875 figures refers to litres per US gallon, this fooling him into thinking he'd more gallons on board that reality.
- The fuel gauges, whilst not to be trusted, were indicating well under what they should. If he flown that aircraft on previous occasions, he'd probably have a feel for whether that was 'right' or not. He should have given up, or at least gone back to square one and re-thought things, at that point.
Sent from my high horse
Personally, if I'd found that the intermediate airfield had no fuel available I probably wouldn't even have done the flight with margins like those - intentionally flying an aircraft with such a low fuel state would make me very uncomfortable indeed even if the calculations had been done correctly, there is just too much which could go wrong and too much at stake if it does!
Well, at worst pilot X's calculations led to him working in US gallons - which is probably what the POH used.. I'm not so certain that any of these calculations landed him in the soup directly. He flew for, at most, 1 hour 10 minutes, so 7.5 usable US gallons should have been enough for an hour and a half of flight assuming 5 gph. That's too small a margin for safety, but he would still have made it.
The figure of 7.2 lb/gallon is Imperial, so had Pilot X used the correct figure of about 6 lb per US gallon in his weight calculations, he would have taken about 10 minutes more fuel with him and this might have been enough to get him home. However, this would have still been cutting things extremely fine and would have left no margin for go-arounds or diversions.
The question is: whether those 5gph consumption were us/imperial and whether they were valid for that aircraft, flown in that fashion. Figures seem to vary on the internet, but some webpages claim that c150s have been known to consume up to 10gph and as little as 4.6 gph.
That figure of 4.6 gph:
- Would be for a new engine.
- Probably assumes the aircraft is cruising and leaned.
In practice. most c150s have old engines, and whilst pilot X wasn't doing circuits he did do 3 take-offs and at least 2 landings, as well as engine run-ups. These would have used significant additional fuel, relative to a straightforward 1 hour 10 minute flight.
Much as people deride whizz-wheels, mine has conversions for both imperial and US gallons which may have served as a reminder that both types exist. However, I think the fuel-units issue is a relatively minor issue here. I think Pilot X's ultimate mistakes were:
1) to knowingly cut things far too fine
2) not to refer to appropriate sources for parameters such as fuel consumption and conversions. Probably the POH would have given figures for different throttle settings which may have alerted him that 5gph was unrealistic, but really if you're going to cut things that fine you should measure figures for the individual aircraft you're flying.
3) not to be flexible with his planning, except when 'forced'. For example, he could have asked his friend to come to the bigger airport when he realised that the weight was so critical. He could have taken a taxi back home from the farm strip, and probably still made it home in time to have supper with his girlfriend. Rescuing the aircraft from the farm strip would have been a lesser hassle than rescuing it from wherever else it ended up.
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