C152 Misbehaving

[Mr Cessna]

C152 that I usually fly began to leak a copious amount of fuel from primer and underneath the primer. It just so happens that the primer on the 152 is located adjacent to the master electrical switch and with fuel dripping out it made me feel very uneasy. (may just take one spark). Anyway, I tried to lock the primer but after several tries realized that it wouldn't lock. so after a few I walked back to the club and decided to pack it in for the day as the clubs other Cessna was also broken.

My questions are:

what is the cause of this problem, could it have been very serious and finally if the primer had of locked would you have flown the aircraft?

Thanks

[BackPacker]

Flying with a fuel leak of any kind is a no-go in my book. Doesn't really matter where, when and why it happens. If there's fuel in places where it should not be, there's something wrong.

[Katamarino]

I'm with Backpacker on this one - fuel should stay where it's told to! I'd have been out of that aircraft faster than you could say "hmm, I wonder what heppens if I turn on the master".

[Pilot DAR]

Yes, I agree that if a primer is leaking in the cockpit, that is unsafe, and should not be flown. There are several possible causes for a leaking primer, all of which are maintenance issues, and beyond the scope of the pilot.

Letting alone the obvious fire risk of fuel in the cockpit for a leaking primer. If it is leaving out fuel sometimes, it is probably leaking in air at other times, and this could have a small affect on the operation of the engine, depending upon where the primer nozzle is in the induction system.

The aircraft in that condition is not airworthy, and should not be flown. Suggesting mitigations as to flying it in that condition is not a great idea. If you're stuck in the bush, and have to get the plane out to save a life, perhaps, but to just go 'round the patch for an hour, that plane should not be flown until repaired.

[KandiFloss]

That sounds really dodgy. You were absolutely correct not to even think about flying that aircraft. Fuel leaking anywhere is bad, but next to the master switch . Also if the primer would not lock, the chances are that it would be allowing extra fuel into the carburettor causing a rich mixture, which could lead to an engine failure.

You said that the other C-152's were all sick too, doesn't sound like the maintenance is good at your club. Be careful .

[AJ1990]

I had this happen to me when I started training and was going to fuel up the plane one day. When priming, it squirted fuel all over my hand much to my horror and I decided best not to start the engine and walked back to the clubhouse. Told the instructor who informed me this was normal and happens occasionally. We went on to continue our flight. Slightly concerned by my instructors lack of judgement right now.

[hatzflyer]

This is a common fault on these aircraft , its the o ring that goes. its a five minute fix for the engineer.

Usually the o ring swells a little before it gives up and that makes it difficult to lock the primer. At this stage, if you can lock it , it won't leak and is OK to fly if you have to but should be reported so that it can be fixed before the leak happens.

[Pilot DAR]

its the o ring that goes

Generally yes, I've changed quite a few.

However, it could also be a broken primer line in behind the panel. If that is the case, obviously a different repair is required, and simply locking the primer in would not necessarily fix it.

If we're discussing what defects are acceptable for dispatch of an aircraft which does not have an approved minimun equipment list, the answer is actually no defects (save for the ever mentioned burned out nav light for day flying).

Private pilots flying simple aircraft on routine flights should not be dispatching with undispositioned system defects. If the maintainer declares and placards a system inoperative in accordance with an approval, and dispatches the aircraft, that's fine. I very much doubt that a maintainer is going to dispatch an aircraft with a fuel system leak...

In the absence of written (log book or placard) disposition of a defect, the pilot who flies a defective aircraft is taking all of the responsibiliy upon him (or her (see "fraternity"!)) self. What will you the pilot say, when the authority asks after the "event"; "Mr./Ms. pilot why did you fly the aircraft with the known unservicability?". Unless your answer is "to fly all of these women and children away from that tornado to save their lives", you might be answering more questions you don't like!

[jxk]

To me, it has always seemed silly to bring the fuel through the firewall to the primer. I notice on some of the later aircraft this has been resolved by electrically operating a primer inside the engine (carburetted) bay itself. However, I suppose statistically the old primer system has not proved too much of a problem.

[hatzflyer]

PD, I don't disagree with anything in your last post but I did make a point of saying that if the o ring swells it makes the primer hard to lock. If the leak is (as you pointed out ) coming from a line behind the primer it would not make the primer hard to lock and indeed locking it would have no effect on the leak.

[Mike744]

Do O rings swell? I thought the rings either/or wear or harden with age. A device that brought fuel in close proximity to the firewall and electrics should at least have the O ring seals changed before any significant wear or hardening.

[Pilot DAR]

O rings very certainly can swell. Particularly if the O ring composition is not compatible with the fuel. In plunger primers, swelled O rings are usually first noticed as the primer being rather stiff, is it has more swelled surface area in contact with the bore. Primers also go stiff because they need lubrication restored with gasoline proof grease, but that's another story.

Primers are easy and accessible to maintain, so there really is no excuse for flying with one that's in poor condition...

[Mike744]

Thanks, that I didn't know. A bit of googling shows "swelling action of the lubricant as fractions become absorbed into the elastomer"

[Maoraigh1]

The C152, C172, and Pa28 checklists I have all have the master switch on before priming. Should the master be left on, aircraft evacuated, and battery disconnected? With fire service present?

[Pilot DAR]

Should the master be left on, aircraft evacuated, and battery disconnected? With fire service present?

My opinion is that if fire is a risk, having or leaving the power on increases that risk. Asking passengers to leave promptly and safely would be a really good idea. You should take reasonable measures to secure the aircraft and minimize risk, unless you feel you are in imminent risk of harm.

Though theoretically possible, a meaningful spark at the master switch itself is unlikely. It only energizes a solinoid (aka contactor) which is where a spark is much more likely if it's going to occur. That solinoid is sealed, so very unlikely a source of ignition. Disconnecting batteries is best left to maintenance staff. Doing that wrong really raises the risks that you are trying to minimize.

[Deeday]

The master switch energises a solenoid? I thought that was the case for the starter switch, because of the beefy current absorbed by the starter motor.
When you close the master switch contact, I suppose you typically have little or no electrical load at all. A relay for that sounds a bit of an overkill.

[jxk]

DD Usually, with exception of some earlier types of aircraft,the master switch grounds one side of a relay (normally adjacent to the battery), thus energising it and applying power to the aircraft. The starter relay works in a similar way and cannot be activated until the master switch is turned on. On your typical C172/PA28 there is also a third relay which operates when ground power is applied, this is designed to isolate the battery on start up. You will also find a diode across the coil of the relay to ground any high voltage caused by the collapse of the coil when master turned off.

[Pilot DAR]

because of the beefy current absorbed by the starter motor.

DD, think of it this way; you can't operate the starter until the master switch is on. So the master switch also controls the starter (though don't ever do it, you could hold the starter engaged with the master off, turn the master on, and get cranking there and then. That's why it is prudent to consider the prop swing area when you turn on the master, just in case of the very remote possiblity of a stuck starter soliniod).

The starter motor is connected to the battery (or ground recepticle) by wire to size of a finger. That wire does not go up to the back of the master switch, so you know there's a solinoid (relay/contactor) in that circuit somewhere. It's usually very close to the battery to keep the runs of big expensive heavy wire short.

As stated, generally, the operation of the master switch is to ground the soliniod coil, rather than to energize it. It is for this reason that the soliniods used in these circuits are special, and not interchangeable with those typically found in automotive applications.

But, I have drifted this from a fuel leak thread. Fuel in the cabin is a necessary design element of every powered airplane I can think of. Leaking of fuel at all, much less into the cabin, should not be accepted.

[jxk]

And of course no one has mentioned the fact that the fuel selector is also in the cabin and can be be prone to leaks in the same way as the primer, with the fuel gathering under the floor. By the way do you turn the fuel selector to OFF in the 152? If I remember correctly there was an instruction to do this because if it was not used often the fuel selector would stick and eventually be difficult to move. This could prove a problem if an engine fire was detected.

[Pilot DAR]

The fuel valve/selector could be a source of leaks, though I have yet to ever see one leak. The only part which requires a seal (O ring or packing) rotates, rather than slides. Thus they seem much less likely to leak. They certainly should be exercised. Though few 150's have them, when possible it is wise to drain the fuel valve sump under the belly. On my 150, it is the only place that debris accumulates in the fuel system. I have a sump drain there, and check it regularly.

From a design perspective, it is the possability of a leak, and fuel fed fire in the cabin, which is a major reason to not have fuel pumps in or very close to fuel tanks supplying the engine(s). You just don't want fuel under pressure in the cabin.