Alpi pioneer rotax carb bowl drip trays cracking

[DrZoos]

Hi guys just wondering is anyone else running carb drip trays on there rotax and are they cracking?

 

We have had both crack. Then they gave us a new design which have also cracked...

 

Personally i think they are pointless as i cant imagine any fuel sitting in the tray anyway and ive not seen them on other rotax aircraft...

 

Whats your thoughts comments ta.

 

 

 

[rgmwa]

Vans include drip trays for the 912ULS in the RV-12. No drains.

 

rgmwa

 

 

[flyhi]

Tecnam's have drip trays with drains.

 

 

[Downunder]

No drip trays on my Foxbat....... Do they have a drain and hose? Would only be useful for a leak while not in operation, stopping the fuel covering the exhaust I would think?

 

 

[Head in the clouds]

Personally i think they are pointless as i cant imagine any fuel sitting in the tray anyway and ive not seen them on other rotax aircraft...

Whats your thoughts comments ta.

They're very far from pointless. If you have a jammed float valve, which is surprisingly common, often caused by a tiny piece of fuel tube sliced off when fitting the hose to the hose barb - so is on the downstream side of the fuel filter - the fuel pump(s) will make the fuel overflow the float bowl and, in the absence of the drip tray (with drain), the fuel will run onto the exhaust header. At any power setting above about half throttle the header is sufficiently hot to ignite the fuel instantly. A fuel fire in-flight is no fun at all.

 

In the photo it appears that your exhausts are lagged, that's great for not radiating heat to other components, cowling, wiring etc, but also means that the headers will be even hotter than otherwise and so will ignite fuel at lower power settings than if they were exposed.

 

I would suggest having those cracks welded before they progress further. One advantage of doing so is that they will be less likely to crack again because the heat of the welding will anneal the aly and thus reduce its temper making it less hard and brittle, and hence more crack resistant.

 

The fact that they have cracked like that in the first place indicates that you have a vibration resonance on the part and so you need to damp it. You could put some lines of high temp silicone (the red stuff - it's heat and fuel resistant) on the back of the part, or add a cable-tie from the tray to something else nearby. I would suggest not using a spring as it can excite the condition rather than damping it.

 

 

[aj_richo]

Earlier this year I had an inflight carb leak on a P96 Tecnam. One of the float pins on the early Bing made its way out of the bowl.. leaving a hole for fuel to drain out.

 

The tray captured the fuel and dumped it overboard instead of flooding the engine and cowl area.

 

I was flying it in mildly turbulent conditions at the time, and briefly smelt fuel... didn't think anything of it.. but on reflection I should have!

 

Anyway, on return to the line there was much arm flurrying and chopping motions from the owner when I taxied in, at idle there was fuel freely running from the drain!

 

I think you need them trays!

 

 

[DrZoos]

We have asked Alpi for a new set. 2 nd set under warranty.....

 

We performed 200 hourly today and re tuned carb settings to try to minimise any vibration....

 

Carbs where slightly out....

 

 

[geoffreywh]

the Texans have them too, and they crack.

 

 

[Dafydd Llewellyn]

Hi guys just wondering is anyone else running carb drip trays on there rotax and are they cracking?We have had both crack. Then they gave us a new design which have also cracked...

 

Personally i think they are pointless as i cant imagine any fuel sitting in the tray anyway and ive not seen them on other rotax aircraft...

 

Whats your thoughts comments ta.

Drip trays with drains are quite common on aircraft, if the exhaust is lower than the fuel system. They are one of the means of compliance for FAR 23.863 (and there are similar words in most aircraft design standards):

§ 23.863 Flammable fluid fire protection.

 

(a) In each area where flammable fluids or vapors might escape by leakage of a fluid system, there must be means to minimize the probability of ignition of the fluids and vapors, and the resultant hazard if ignition does occur.

 

(b) Compliance with paragraph (a) of this section must be shown by analysis or tests, and the following factors must be considered:

 

(1) Possible sources and paths of fluid leakage, and means of detecting leakage.

 

(2) Flammability characteristics of fluids, including effects of any combustible or absorbing materials.

 

(3) Possible ignition sources, including electrical faults, overheating of equipment, and malfunctioning of protective devices.

 

(4) Means available for controlling or extinguishing a fire, such as stopping flow of fluids, shutting down equipment, fireproof containment, or use of extinguishing agents.

 

(5) Ability of airplane components that are critical to safety of flight to withstand fire and heat.

 

© If action by the flight crew is required to prevent or counteract a fluid fire (e.g. equipment shutdown or actuation of a fire extinguisher), quick acting means must be provided to alert the crew.

 

(d) Each area where flammable fluids or vapors might escape by leakage of a fluid system must be identified and defined.

 

[Amdt. 23–23, 43 FR 50593, Oct. 30, 1978]

 

 

[DrZoos]

Thanks guys ..for the replies some excellent points

 

Seems strange so many dont have them

 

Anyhow looks like we will just have to keep replacing them each hundred hours or so

 

 

[Guest Maj Millard]

Dr Zoos, the Slepcev Storches had them also...cheap arse alum ones that didn't last too long either. Any drip tray under the carb bowl is a pain when you are trying to remove/refit the carb bowl.

 

If the needle float valve is replaced as required or checked during a 100 hourly it rarely gives any problems. More the case is an engine running rough or a bowl leaking because the bowl and gasket haven't been fitted correctly or the gaskets need to be renewed.

 

Personally I like to put a bit of Locktite 515 on my carb bowl gaskets, they seal a little better and don't fall out onto the dirt when you remove the carb bowl.

 

I do agree that there could be fire risk if the float needle valve sticks and fuel leaks onto hot pipes below. However I don 't believe a hot pipe alone will ignite fuel.

 

I don't have drip trays on the Light wing, however I have extended my compensating/ fuel drain tubes down about 12 inches so that any fuel leaked doesn't go anywhere near any hot pipes.

 

In the attached photo the Pipistrel Virus engine which has all the bells and whistles to make it suitable as a high altitude capable motor glider isn't fitted with any drip trays...........Maj......

 

 

 

[Camel]

I may be stating the obvious to some but Rotax 912's don't like to be left idling as there is a lot of vibration whilst idling and whenever the plane is stopped the engine should be running around 2000 rpm to reduce vibration, I have seen people let them idle on many occasions.

 

 

[geoffreywh]

make a triangular "doubler" riveted to the tray that helps support the tray, they don't crack after that

 

 

[Dafydd Llewellyn]

A hot pipe will ignite fuel at a little over 300C. If it will melt electrical solder, ignition is likely. Oil ignites at a considerably lower temperature.

 

 

[M61A1]

Try polishing the radius where it's been cracking when you fit new ones, it appears to be just a stamped edge. a couple of $ worth of flapper wheel to polish up the edges should go a long way towards preventing cracking. The doubler idea is worthwhile, as is Maj's vent line extension to get all overflow dumped outside the cowl.

 

 

[Guest Maj Millard]

Also be carefull with elect boost pump usage......very easy to overpower the float bowl if left on while idling, which can flood the bowl. My Facet boost pump is the lowest available...something like 1.5- 3.5 psi max.......

 

 

[DrZoos]

Mine runs at 0.31-0.33 all the time with or without booster pump...

 

 

[Bruce]

"The fact that they have cracked like that in the first place indicates that you have a vibration resonance on the part and so you need to damp it"

 

Agree with Head in the Clouds, there is a vibration issue and this must be solved first. Balance the prop and the carbs! When was the prop last balanced with strobe or vibration analysis equipment?