Best of luck Aveling... It sounds like you won't really need it, which has to be a good thing.
I've been flying my putt-putt aircraft recently. It doesn't have a fuel pump, gravity does that job, but I still had a fuel delivery problem the other day: the carb float stuck closed and shortly after start-up it unceremoniously quit dead at idle. Hmmmm, I thought, that's a first. Eventually it was sorted out - a tap, tap, tap on the carb body did the trick. I was happy in the realization that it can't stick closed unless it is closed. That only occurs when you shut off the engine, meaning a float being stuck closed will almost certainly cause engine failure only right after start-up.
It was still a bit uncomfortable to taxi out and take off over the houses but there was no problem. I did a long full power run up, not so difficult with only 65 HP on tap, and departed without incident. With light fuel load and no passenger, I climbed out at almost 700 fpm on 65 HP. No problems since but I've learned of something new to watch for.
A suggestion: take the local engineer on the flight
Now that's a cleaver suggestion!!
Sorry to resurrect this old post but I thought I’d bring it up to date for anyone who is interested.
Back in UK the intermittent low fuel pressure behaviour persisted. Many opinions said “they all do that”. A new mechanical pump did nothing, made it worse in fact. The breakthrough was installing a “new” unused stock electric pump, at which point the engine would not run with it switched off! The Mechanical pump could not draw fuel through it. Another “new” electric pump, this time new new and not new but on the shelf, fixed it and many hours later no sign of a re-occurence.
So conclusions: the PA28 design is so marginal that it’s possible to have a mechanical/electrical pump combination where the lifting ability of the mechanical pump is insufficient to properly draw fuel through the electric pump when it is switched off. Both pumps are perfectly OK in their own right. This fits in with the “they all do it” view. Well my aircraft didn’t do it before, and hasn’t done it since. But apparently many do.
So how is possible for a certified airplane to exhibit this behaviour? Answers please, on a postcard addressed ‘FAA’.
The breakthrough was installing a “new” unused stock electric pump, at which point the engine would not run with it switched off! The Mechanical pump could not draw fuel through it. Another “new” electric pump, this time new new and not new but on the shelf, fixed it and many hours later no sign of a re-occurence.
I understand some Bendix/Facet electric fuel pumps (as fitted to many Pipers) have internal check valves or Positive Shut Off valve (PSO), and some do not. Pipers apparently use the version with no PSO and they are externally indistinguishable. Perhaps that was the issue – I am no expert.
http://www.facet-purolator.com/gold-flo.php
http://www.aircraftspruce.com/pdf/2013Individual/Cat13289.pdf
Very interesting. The PSO appears to be an anti -syphon valve which might prevent fuel being drawn through the pump. Could explain the not running at all episode, but not the rest.
So how is possible for a certified airplane to exhibit this behaviour?
Unfortunately I see a lot of this in the avionics business. Once a company is certified, it can bring out all kinds of dodgy products.
I don’t think there is any “second opinion” type overview of designs in GA – mechanical or electronic. And even less so decades ago when a lot of “our” kit was certified.
I’d guess a series of inappropriate electric fuel pumps had been installed in the aircraft, starting before the incident, until eventually the right one was used. All but the last compromised the function of the mechanical pump and its delivery path, because they were the wrong part. They all look the same, but AIUI different models of what appear to be the same pump are not directly interchangeable. Judging by the catalog listing, there are versions with either, both or neither Positive Shutoff Valve and Check Valve.
It sounds to me as if you need to check that both pumps are the correct part number according to the current aircraft parts manual.
I would still check that all fittings are airtight, especially the cascolator seal. Possible also that in problem shooting the mechanics have fixed airleak without intension.
