Mike wrote about an engine failure and forced landing that's been
attributed to "vapor lock."
Mike,
First, I must emphasize that I'm not a mechanic and you should research
very thoroughly any of this information yourself.
Quick research finds a reference showing 100LL has a vapor pressure of:
5.5-7.0 psia (Reid VP) @ 100°F.
Jon Hines used to give us a pretty well researched speech on avgas
versus mogas. One point I remember from it was that while avgas had a
tight range of Reid Vapor Pressure (RVP), mogas was allowed a higher
range from around 7 in the summer up to around 15 in the winter.
Using winter blend mogas in the heat of the summer might allow vapor
lock in situations where avgas or the proper seasonal blend of mogas
would not.
Marathon Oil has a paper online that discussers RVP regulations
(particularly summer blend):
http://tinyurl.com/2begqot
Still, my early pilot training taught me that you need to have a high
point in your fuel lines where the vapors would collect in order to have
vapor lock. Otherwise, the fuel turning to vapor would result in the
vapor simply moving back to the tank or forward to the carb.
Did you have a high point in your fuel system other than your nose tank
or carb? If so, you should fix that. One advantage of our header tank
is that it allows reliable gravity feed and allows fuel lines to have a
constant downward angle from the fuel tank exit to the gascolator then a
steady upward angle to the carb, thus preventing vapor lock.
Even then, doesn't vapor lock normally occur on the ground when a hot
engine has a chance to heat the engine compartment and fuel in the fuel
lines with the fuel collecting in the high spots? I don't think I've
ever heard of vapor lock occuring while in flight.
I'm highly suspicious of the "vapor lock" theory as a true cause for
fuel starvation when flying along.
Other causes of intermittent problems I've heard of include:
* sloshing compound glob drifting over the fuel outlet of the nose tank
* fuel line that collapses on the inside while looking fine on the
outside
Could a sinking carb float cause engine stoppage? It seems to me that'd
cause too much fuel to fill the chamber rather than too little.
I'll stop my very NON-expert speculation and stand aside for more
knowledgeable answers.
Ed
On 2010-08-12 12:25 AM, Mike Harrison wrote:
In Montana, we have three grades of auto fuel: regular, midgrade, and
premium. The first two may contain ethanol (I have never found any
ethanol at my local Conoco station), but by law the premium fuel MAY
NOT CONTAIN ethanol. This makes it easy to avoid ethanol.
I recently landed my Ercoupe in a hay field after an abrupt engine
failure. Praise the Lord, I was totally uninjured and used no bad
language, but my beautifully restored coupe suffered a collapsed nose
gear due to a deep rut in the field near the end of the landing roll.
It will be a while before she flies again. Need a rebuildable nose gear!
No mechanical problems discovered. All fuel screens were perfectly
clean. No water in fuel. Ambient temperature was 80 deg F and CAVU
with low humidity. The O-200 engine was operating smoothly at 2400
RPM. Carb ice not suspected. The fuel mixture was about 80% mogas and
20% 100LL. Wing tanks nearly full; nose tank full. Official cause:
"Undetermined." The NTSB rep and my mechanic are suggesting the
problem was possibly "vapor lock." Both of these fellows "hate" mogas
and think the mogas STC should not exist.
I have read thoroughly and feel that EAA has done an excellent job
testing mogas in the O-200. According to Continental, the O-200 was
originally designed to run on fuel with a minimum of 73 octane and
little or no lead.
Question 1: Does Mogas have a higher vapor pressure than 100LL? Is it
more prone to vapor lock than 100LL?
Question 2: Has anyone ever installed a boost pump between the nose
tank and the carburetor? Is there an STC? This pump would be similar
to the electric fuel transfer pump used on some Ercoupe models with
O-200. Many modern AC use an electric boost pump in addition to the
engine-driven fuel pump. E.g., Piper Tomahawk, you flick it on during
critucal phases of flight, such as take-off.
Question 3: Does Marvel Mystery Oil help prevent vapor lock? What
other ideas would help prevent vapor lock?
BTW: I have flown this Ercoupe at -22 deg F ... the cabin heater would
have been adequate if I didn't have to keep the windows cracked open
to keep the wind screen from frosting up! Do you want to be warm or
"see and avoid?" No vapor lock that day!! Also, I have flown it to
15,400' MSL on mogas. No vapor lock that day either.
Sincerely,
Mike Harrison
Whitehall, MT
(406) 560-5370 Verizon cell
