Choke point

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Internal diameter of the MIL 6000D hose (right) has shrunk to about 50% of the the same MIL 6000D hose after overnight exposure to Mogas
Internal diameter of the MIL 6000D hose (right) has shrunk to about 50% of the the same MIL 6000D hose after overnight exposure to Mogas

A mismatch between fuel and hose can result in engine stoppage, reports Roger Alder

Flexible rubber hoses are like the blood vessels and arteries of your aircraft—carrying fuel, engine oil and hydraulic fluid—and just like the arteries in the human body, these hoses can suffer hardening, become varicose, and suffer anaphylactic shock. Rubber hoses carrying gasoline in general aviation aircraft are not even under particularly high pressure. What could possibly go wrong? Plenty!

Hardening of the arteries occurs over time (age!), and when extensive fat build-up on the arterial walls becomes calcified.

In aircraft, engine oil hoses become hard (and brittle) due to the effect of the heat in the engine compartment and the heat of the engine oil on rubber and to a lesser extent, PTFE. Exercise does not help. Stents are not an option. The only thing to do is surgically remove the old hoses and put in fresh new ones—regularly.

Varicose veins in aeroplanes were first brought to public attention around 1998, when a faulty batch of rubber was used and in another case, a particular breed of rubber hose was found to go soft and leaky, particularly when under the influence of low-lead Avgas and Mogas.

Then manufacturing faults saw a spike in hose failures in the 1990s. In 1997 CASA issued the airworthiness directive AD/HOSE/5 in response to a batch of hoses used on Cessna aircraft (and their licence-built Reims versions), which were in the words of the directive ‘prone to internal collapse’.

‘The result of such a failure in fuel, oil or hydraulic lines could be catastrophic and so the defective product needs to be identified and removed from service,’ the directive said.

AD/HOSE/6, of 1998, applied to Beechcraft aeroplanes that had been equipped with a bad batch of MIL-H-6000B fuel hoses.

Like the Cessna hoses of AD/Hose/5 they were prone to internal collapse, thereby blocking fuel supply. The consequences of this need hardly be elaborated. Both hose Directives were responses to faulty batches of hoses, and interaction of the rubber hose with fuel.

But another hose health issue has re-emerged, where the problem is the interaction between the hose and some of the new breeds of fuel used in aeroplanes.

Anaphylactic shock

In this case, MIL DTL 6000 D hose has been found to swell shut in the presence of Mogas. With the price of automotive unleaded petrol, and Mogas, in particular, coming down dramatically in recent months it’s an issue worth re-exploring, to emphasise its moral: penny wise can be pound-foolish.

One cluster of these failures was reported by a sharp-eyed LAME from Orange, NSW. He noticed common elements in engine failures across a range of aircraft, ranging from weight-shift trikes to a Cessna 172. In all cases the aircraft had experienced fuel starvation while using MIL-H-6000D hoses in conjunction with Mogas.

This problem has happened before, with one example being the operator of a Kitfox sport aircraft in the US, who in 2002 told the Federal Aviation Administration how a short length of MIL-DTL-6000 rubber tubing used to join two metal fuel pipes had ‘swelled shut’ causing an engine failure in flight. As the pilot of a low-landing speed STOL aircraft capable of putting down without further incident in a farmer’s field the Kitfox pilot did not seem unduly concerned. If only forced landings at short notice were such a non-issue for the rest of us. Again the conjunction was between MIL 6000 hose and automotive fuel, which the Kitfox used exclusively. The tank on the other wing used a different type of hose, and tellingly, it did not fail.

Transport Canada notes that using Mogas in fuel systems designed for Avgas can cause blockage of fuel sump drains, and deterioration of O-rings in fuel filters and fuel primers. Mogas is also more likely to produce vapour lock at high temperatures and carburettor icing at low temperatures than the Avgas, which is specifically formulated for aircraft piston engines.

Engine and hose manufacturers are wary of Mogas. For example, Lycoming Service Bulletin No. 509 (Aeroquip Service Bulletin No.AA135) strongly recommends that Aeroquip 601engine rubber compound fuel hoses have regular periodic inspections, and imposes a recommended replacement life of just two years when used with low lead Avgas or Mogas.

That two years might seem just a mite generous when you consider that rubber compound hose manufactured to meet MIL-H-6000 D (now MIL-DTL-6000D Amdt 2 01 Oct 2014), can suffer nothing short of anaphylactic shock when in contact with Mogas, to the extent that the hose can quickly soften and swell causing the internal diameter of the hose to shrink until completely closed, in a matter of hours, severely restricting and cutting off the flow of fuel, and causing engine stoppage.

Although some suppliers of MIL DTL 6000 D hose may state that hose made to the specification is ‘suitable for fuel for aircraft engines’, a good question to ask is ‘which engines?’ and another is ‘what fuel?’

It is most unfortunate that so many people have incorrectly assumed that where the MIL standard broadly states in the opening pages, that the hose is intended to carry ‘fuel for aircraft engines’ that this means any fuel, including gasoline. It does not. You don’t have to be an avid page-turner of MIL Standard jargon to find that the reference fuel identified in the MIL Standard is US Military kerosene, not Avgas, or Mogas. You might like to read that again.

While MIL DTL 6000D hoses seem to cope reasonably well with the very precisely formulated Avgas, the much more variable nature of Mogas appears to be a problem.

Another way to find yourself with distinct lack of noise and an altimeter with needles that are rapidly moving anticlockwise, is to fill up with Mogas (because you can, you are the proud owner of an authentic Flight Manual Supplement which says so!). Unfortunately, because you wanted more range, you bought and had installed another STC to fit an auxiliary fuel tank. Seems at this particularly awkward moment, low on fuel, you can’t seem to get the fuel out of the aux tank to somewhere where it might be just a little more useful. Problem being, that you didn’t notice the word “Avgas” as the fuel to be used, in the Flight Manual Supplement that came with the aux tank STC.

Guess what hoses were supplied with the STC?

Fuel and aircraft fuel system component compatibility are a complex issue that will not go away. Lack of attention to detail has resulted in many embarrassing and downright painful moments in recent aviation history. Let’s avoid fuel and hose-induced sudden silences.

Keep an eye out for CASA AWB 20-006 Issue 2 (published soon) which explores more aircraft fuel and oil hose safety issues.

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2 COMMENTS

  1. My comment to the Aviation experts

    I suspect the problem of swelling of the fuel hoses is caused by the use of “premium” car fuel which contains the additives of benzine and toluene. This is used to increase the octane rating. “Standard” car fuel does not contain these additives. Benzine and toluene are also highly carcinogenic. Also the use of ethanol additive in car fuel will cause severe swelling of rubber, even a small trace of it in car fuel will find its way into rubber.

    Modern motor cars do not have this problem with fuel hoses and O rings. They use synthetic material. Aviation is way behind the times in technology.

  2. Ian:
    How recently did “modern” cars move away from rubber hoses and “O” rings? A few years ago I noticed sticking “O” rings in a C172 primer soon after avgas was eschewed for premium (ethanol free) mogas. DC4 corrected the problem for a short while, but it returned. The owner stopped using mogas and returned to 100LL and the problem never reappeared. Happenstance or consequential???

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