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Partial engine failure

I mentioned on another thread that my engine stopped a little while back, and that I’d post more about it when I’d worked out what happened. Finally, I think I have.

The background is that I bought a rather neglected Turbulent a few years back. It’s a wooden aircraft with a VW engine in it. Tailwheel. Similar performance to a C152 but with handling that is a lot crisper. Having realised that I would never have enough time to build an aircraft this side of retirement, I wanted something that was immediately airworthy but could be spruced up whilst flying it and I guess I haven’t been disappointed in the latter respect!

I went out one warm summer day for a trip from Welshpool to the coast and back. This involves crossing some fairly inhospitable terrain, and right in the middle, my engine stopped. Bizarrely my first thought was that it must have inhaled a bumblebee. I pulled on carb heat and after a second or two it burst back into life, settling into its usual rhythm after a few more seconds of rough running. I waited for a few more seconds but whatever the problem was, it seemed to have cleared. I considered turning back, but figured I might as well continue to the coast: if it gave me any further trouble I could fly up the coast to Llanbedr. Luckily it was a nice day so I climbed, though I descended again when the terrain became more benign. Even on a summer’s day, an open-cockpit at 6000 feet can get chilly.

I spent a little while flying around Aberystwyth looking for a lost model rocket, then flew over Ynyslas and climbed again to 6000 feet for the trip to Welshpool. The engine had run smoothly for about 40 minutes since the last hiccough, and on a warm humid day I decided it had probably been carb icing, though if I’m honest with myself I didn’t completely believe that was the explanation. It was lucky that I chose to climb – when I was due south of Lake Vyrnwy the engine started to run roughly again – stopping then starting then stopping again every few seconds. I checked the fuel valve, carb heat, oil pressure etc… again – nothing seemed to make any difference. The problem was gradually worsening. Rather than skipping a few revs then starting up again as sweetly as before, the engine was cutting for half a second, then a second at a time. I persisted for a minute or two but it was clear that I was losing altitude and would not be able to make Welshpool. Neither was it a comfortable ride. I wondered whether any of the farmers far below were aware of my plight – even though I was very high, perhaps someone could hear my coughing engine?

I decided I’d better look for a field. Luckily there was one right below me. In Mid-Wales one does not suffer the tyranny of choice when it comes to fields. There was one moderate sized greenish looking field below me and one in the distance – it looked a bit bigger but I wasn’t sure that it was within gliding distance, even from 6000 feet. Lazily I had lost awareness of my exact position, so I spent a while working this out so I could plan my mayday call. I changed to 121.5. Oddly, my voice sounded perfectly calm as I listened to myself make the call. ‘Mayday, Mayday, Mayday, G-AT?? is a Druine Turbulent with 1 POB. I have a rough running engine intending a forced landing. I am currently about 5 miles south of Lake Vyrnwy descending through 5000 feet’. There was silence. My next call was a little less formal. ‘Mayday, mayday, mayday G-AT??… Anybody?!’ I decided I was on my own and gave up on any further calls. I put my gloves on and silently wished I’d bought a helmet.

The rough running continued to worsen… It was tempting to keep what little power I had and delay the inevitable, but surely it would be better to crash with a stone cold engine. I throttled right back, at which point the engine burst into life again. I eased the throttle forwards and found a point at which I could keep it running smoothly. I still wasn’t maintaining altitude, but I decided to strike out for the distant field with a view to returning early if it looked doubtful. To my delight, I found I was making reasonable progress, though I had to watch my airspeed as there was a temptation to slow behind the drag curve – Turbulents typically have no pitch trim to help with this. As I progressed, I diagnosed the problem. Presumably the engine was not getting enough fuel, causing it to cut out intermittently. When I took off, the level of the fuel in the top of the tank was considerably above the level of the fuel pump, but having used about 1/3 of my fuel there was no longer sufficient head of fuel to overcome any deficiencies in the pump. But why then that first hiccough whilst the tank was relatively full?

The valleys began to open out and the terrain gradually became more hospitable. From this point on, I was never out of gliding range of at least a moderately plausible field. My altitude was dropping… 4000, 3000… at 2000 feet I started to seriously consider choosing to land in a field again but I also knew I was getting close to the airfield. I called them to report that my engine was running rough and I was going to make a straight in approach. Finally I saw it! I reached the airfield with about 1000 feet in hand. Out of the corner of my eye I saw the air ambulance blades spinning. I made sure I was well above the glideslope then throttled back and sideslipped in to land. Inevitably I arrived too fast and bounced. I arrested the bounce with the elevator, knowing that I couldn’t use power to save me, and somehow I converted it into a bouncy wheeler-landing. Not elegant. I still had enough power to taxi back to the hangar so I did so, before shutting down. Then I called a taxi so I could catch the train to pick my son up from school – having flown very slowly for the last half hour I was at serious risk of missing it. Then I called D&D just in case somebody had heard my Mayday call (they hadn’t).

As ever, there are two questions: what could I have done better, and what caused the problem in the first place?

The temptation is to think that because my flight had a good outcome, I handled the emergency correctly. In some respects I think I did do well, but I’m not sure that I actually chose the wisest course of action. Might it have been safer to have made a precautionary landing than to attempt to return to the airfield? Perhaps, but on the other hand Turbulents have small wheels and are prone to turning upside down on rough ground. I think one could argue this either way. In retrospect perhaps I should have flown straight back to Welshpool at the first hint of engine trouble, but that same engine trouble had cleared promptly with carb heat and it was certainly the sort of day when one might have expected carb icing – to which Turbs are prone.

My mayday call was unconventionally structured. I would have liked to say that at least it had everything in it, but of course I missed out the fact that I was a PPL and I neglected to state my heading. When my Mayday call failed I might have tried turning my handheld radio horizontally – perhaps this would have given it enough range to reach any airliners high above. After my Mayday call failed, I think I should have probably broken out my PLB beacon which is attached to my lifejacket. A bad accident might have incapacitated me so it would have been better to activate it at altitude, well before I was having to start planning glide-approaches.

The best thing I did, of course, was to gain altitude when I was crossing the rough bits of the middle of Wales. This for two reasons: one that even with 1000 feet less, I wouldn’t have made it back. Secondly, it gave me time to approach things like Mayday calls and troubleshooting in a fairly leisurely way. The truth was that I could have been more organised. Why was I not wearing my gloves to start off with? Why did I have to spend so long working out where I was?

I’ve been back to see the aircraft twice – the first time I was armed with a fuel pressure meter and a spare fuel pump – to my surprise the pump and engine were working perfectly and disconcertingly, I couldn’t find anything to fix. On the second occasion (yesterday) I decided to bring back the fuel pump and carburettor home for further investigation.

Behind the needle valve, I found this:

Actually that big bit of brown material isn’t the exact same one that was there – the original jumped away before I could photograph it so I put a similar sized bit there to illustrate what I presume caused my fuel starvation.

Inside the needle valve, you can see there’s more debris; some of which is brownish. There’s also a small fleck of rubber from a fuel hose.

And taking the fuel pump apart, one finds the gasket from which it came:

Amusingly I find this on the internet from the previous owner, who claimed the engine had never given him a moment of trouble when he sold the aircraft:

“Yesterday I lined up to take off and it just stopped dead! I got out and swung it again and it ran for about 5 seconds and stopped again. Next time I was a bit quicker running round to pump the carb and managed to taxi back to the hangar, all the way it was spitting, banging and misfiring and would not get above 2000 rpm. I stripped the carb, got a compressor to blow out every orifice I could see, put it together again and fired it up.
At least it now ticks over like a sewing machine but when I go thro 1500 it just wants to stop. If I force it thro with pumpings it will get to around 2400 but it feels like its misfiring and running very rough.
It has a slow running adjustment which sets how much the throttle will close and one mixture screw that controls both slow and fast running. I started it at 1.5 turns out on it and standing behind the running engine I spent hours turning it richer and weaker in 1/8 turn increments to get it to run above smooth above 1500….BUT NOTHING WORKS TO MAKE IT RUN SMOOTH EVEN ABOVE 1000 RPM!!! I thank God it stopped on the runway when it did, another few minutes and I’d have been in the air looking for somewhere to crash safely!!”

Last Edited by kwlf at 06 Oct 01:19

Good job nursing her back! Tough choice – an outfield landing you never know what you get, but at the same time, you don’t want to put yourself too low over built up areas trying to get back to an airport either. I think you did the right thing.

What a great report, kwlf, and many thanks for posting it.

You did a brilliant job and I would hope to do as well if it happens to me one day.

On a mayday call in the UK, D&D get a triangulation fix (if you are high enough) instantly and there is no need to report your heading or your license type etc etc.

Yes it is hilarious what you find on the Internet isn’t it? As someone who is in possession of a vast number of stories from pilots (which cannot be published) and then gets contacted by successive owners who were told nothing, I am not at all surprised In GA, disingenuity rules… and doing a google on the aircraft reg alone usually digs out a ton of stuff like where it has been, with photos, and from the pattern you get a pretty good indication where it almost certainly has not been (but claimed to have been). Unfortunately this also prevents a lot of stuff being posted which should have been.

That gasket is in a terrible state and I wonder how old it must be. Normally there are filters in a fuel system whose job is to prevent this sort of thing but you cannot guard against debris from something downstream of the filter. However that gasket material should not be downstream of any filter. That gasket has either been eroding for many years, or it is homemade. It would be better to make one out of (the right type of) rubber.

Shoreham EGKA, United Kingdom

Yes, there are three fuel filters upstream of that one, and none downstream. It’s quite sobering to think that all your fuel goes through that tiny channel.

The aircraft first flew in 1968 but there are pictures of it fitted with an engine that are older than that. I suspect the gasket was over 50 years old. There was an inner tube dated 1975.

A friend of mine who flies a BE17 Staggerwing from 1944 had an almost identical problem. A cork gasket in the fuel selector had deteriorated and blocked the fuel line. Forced landing in the field, no injuries but some damage due to vegetation and gear-up landing.
All items in the fuel system are critical, be it fuel caps, fuel selectors, fuel strainers, fuel lines, etc. O-rings are critical low-cost, expensive consequence items.


kwlf wrote:

I did of course search for the aircraft and seller before buying the aircraft, but found nothing of note.

What’s this about the seller and his responsibility? You had debris in the fuel system which is a very common problem. I’ve had it on many vehicles, airplanes included. A complete inspection of the fuel system from the tank to the engine injector is part of regular maintenance. It’s absolutely normal to find old gaskets that need replacement.

Continuing the flight after a momentary loss of engine power was a mistake. If you accept such issues with your airplane without immediately returning to the field for a thorough analysis, you risk surprises like you had. You should not assume carb icing until you’ve inspected the fuel and ignition system and found everything to be in order. If carb icing is a common phenomenon outside the typical carb ice situations, a carb temp gauge should be fitted.

It was a fairly textbook day for carb icing – the right temperature, fairly humid. I wasn’t wrong not to return to the field immediately because that wouldn’t have been the quickest way to get over somewhere I could land safely if I had to. The questionable decision was when I turned back from the coast.

I half regret bringing up the history of the aircraft as in one sense it’s an aside. I had recently replaced some fuel lines but didn’t see any reason to touch the fuel pump / carb for fear that I was more likely to break something than to find something to fix. The carb was taken apart and cleaned about 30 hours ago with no problems evident at the time.

On the bright side, I feel good about having taken it all to bits now, and finding the problem is very satisfying. I’m a tinkerer by nature, but have never previously owned an IC engine and would have been loath to take it all apart without the excuse to do so which this incident provided. My general worry is that the risk of causing some new fault might often be greater than the chances of finding something to fix. In this instance, clearly not so.

Last Edited by kwlf at 06 Oct 11:38

Great story, kwlf.


Excellent report and everything handled perfectly well I would say. I know the underlying problem very well from the Citroen CX I drove for 13 years. Countless times I had to stop along the road (the most memorable being on the St. Gotthard road in heavy snow… luckily I could warm my hands on the still hot engine block) to remove and clean the carburetor jets. I kept the tools required for that in the glove box because they were required so often… And it didn’t matter how often one replaced those cork gaskets, they would always come apart.

Regarding the mayday call, I think you rather told too much than too little. I wonder what any non-native English speaker (as the majority of crew in passing airliners on the North Atlantic route overhead) would understand when a non-Oxford-English native speaker says “Druine Turbulent” and “Lake Vyrnwy”… Myself (we recently had a thread about that) would not even bother to make a call on 121.5 – I would call the first aviation related number of my cellphone-contacts and ask them to tell someone in charge about my problem.

EDDS - Stuttgart

I’m glad you made it home OK. I’ve been in a VW powered aircraft during a similar fuel system related engine failure and although nobody was hurt after the ensuing off field landing, in that case the plane took quite a while to rebuild.

I was curious about VW mechanical fuel pumps after reading your post and found this thread. As usual with air cooled VW parts, there is apparently a wide range of quality in what’s available. It’s possible to rebuild the original pump but it looks to me like if you replaced it with one of these you wouldn’t have to worry about cork gaskets any longer. In no case is cost an issue, everything from a rebuild kit for the standard VW pump to a new pump costs next to nothing.

Obviously the placement of the fuel filter is also something I’d want to consider carefully.

Last Edited by Silvaire at 06 Oct 15:24
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