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First experience of possible vapour lock

This flight was on Sunday 9 August. Our aircraft, an SR22 NA had been sat in the sun at Le Touquet for 6 hours. OAT at departure was 40C.

Whilst in the terminal filling up water bottles and filing the flight plan, the crash alarm sounded. A departing aircraft had suffered engine failure in the climb out but managed to get back on the runway safely, and I overheard some chatter about vapour lock in Mogas powered engines.

The oil temperature was still 140F when we returned to the aircraft. Hot start worked fine and – as per the POH – I kept the fuel pump on for taxy, takeoff and climb up to FL80 on the LYD1Y departure. Mid-Channel and level, with the mixture leaned only to top of green arc (i.e. not LOP) I turned the fuel pump off, mindful of the possibility of vapour lock. Sure enough, the engine spluttered and a loss of power could be felt. Fuel pump back on and all back to normal.

Lessons learned: The OAT at FL80 was 24C – insufficient to have had any significant cooling effect on the hot fuel in the tanks during the short climb. The vapourisation point was further lowered by being at FL80. I will check the engine data from the MFD to see what the effect on fuel flow was (I didn’t check – it was obvious the engine was not happy and needed the pump back on).

In similar conditions I will keep the fuel pump on at all times. Interesting that it creates a single point of failure if the electric auxiliary pump stops working in conditions that are conducive to vapour lock.

Last Edited by MarkW at 12 Aug 16:36
EGSX (North Weald), United Kingdom

Did this happen with AVGAS 100LL?

Where is the fuel pump located in the SR22? In the PA28 it is in the engine compartment so that would not help with vapor lock from the wings until the pump.


Unless the engine is one of the Rotax that are specifically designed to run on Mogas (vapour return lines & ethanol filtering) it will be really daft to run it on Mogas these days where Sea Level DA is 4kft and OAT is past 20C (not even talking 24C OAT at FL80), I am sure everybody in UL/LSA/LAA and microlights will switch to Avgas mix or whole Avgas but who knows? this from LAA is highly informative on the topic and limitations local copy

I had issues with Mogas flying Turbulents, they have WW1600cc Beettle car engines and do take Mogas by default, obviously those cars don’t drive at 6kft, vapour lock & inadequate mixture control were my suspects for intermittent loss of power, in the other hand, I flew a Rotax motor-glider to FL140 on Mogas in a very hot summer and it had no issues (it can glide very well and I heard it takes SP95, SP98, UL91 and even two-stroke oil)

For certified engines, old Lyco & Conti STCs from 1970 have no face value in this topic, I think pilots should know better…

MarkW wrote:

This flight was on Sunday 9 August. The aircraft, an SR22 NA had been sat in the sun at Le Touquet for 6 hours

Were you in CE?
It could be just pure dirt in fuel who knows?

Last Edited by Ibra at 12 Aug 15:49
Paris/Essex, France, , United Kingdom

Yes, Avgas 100LL.

The electric auxiliary pump sits forward of the firewall. It takes fuel from the L or R tank (determined by the selector valve), feeds it into the gascolator and from there to the engine driven pump.

EGSX (North Weald), United Kingdom

Ibra wrote:

Were you in CE

Yep :)

EGSX (North Weald), United Kingdom

What would happen in a cessna with no fuel pump ? Would gravity be sufficient ?


I guess in a Cessna you will always have enough pressure unless you’re flying inverted. But if the bubble is big enough it might work itself upwards in the fuel system and get stuck in some oddly shaped part and still cause a problem.


Unless the engine is one of the Rotax that are specifically designed to run on Mogas (vapour return lines & ethanol filtering) it will be really daft to run it on Mogas these days

A very British view. Half of the German C150/C152/C172/C182/PA28/AA5A/DR400 fleet is operated on mogas, and almost none of them are switched to Avgas during high summer. and nothing bad happens.

Mainz (EDFZ), Germany

As far as I know, nobody in Sweden uses MOGAS. However, AVGAS 91/96 UL is very widespread.


I think you can restart C172 in the air after it sucks air from one of the empty tanks with no fuel pump, just pitch for 90kts (or whatever RPM), mixture & throttle & mags on BOTH (no need for starter) & switch fuel and it will go again

The Mooney POH run dry tank procedure require the use of electric full pump ON but I think it does work without as long as RPM stays higher than 2000rpm in the drop

The question is how much FF and RPM drop you get from that vapour lock or fuel dirt for the mechanical pump & engine to fail? and how much FF & RPM you need to restart?

It’s easy to test this on the ground but you may need to use the mags starter to keep prop turning alive above 1000rpm while fiddling with fuel tank & pump OFF/ON (in the air prop & mechanical pump should turn naturally, unless you feather it or fly slow, so you only have to supply fuel )

Last Edited by Ibra at 12 Aug 18:45
Paris/Essex, France, , United Kingdom
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