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Date: Thu, 14 Jun 2007 16:53:35 -0400
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Subject: Re: Boost switch
From: "bob mackey" <n103md@yahoo.com>
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Rick makes several good points...

> "Lean" is a condition where the mixture is less than
> stochimetric (i.e. LOP).

Ahh... we can wish the English language were so precise,
but it is not. In common usage as well as on this forum,
"lean" is a verb, a relative adjective, and in some cases
relative to a datum as in LOP. I wrote
"Good thing it was lean enough to tolerate that extra gulp of gasoline."
I think you and most of the readers can properly parse that sentence.
In this case "lean enough" means that there is little enough fuel
mixed with the particular amount of air so that adding 4-5 gph
results in a combustible mixture. We have since learned from Jeff that
he was at 32" MAP. That's a lot of air. With that much air, 4 gph
more than the LOP fuel flow, or 5 gph more than the ROP fuel flow
did not kill Jeff's engine. (By "kill" I mean quit producing
flight-sustaining power, but I suppose most of us could have
figured out what I meant by that ambiguous word also, given the
context).

> I'm also a bit confused in your "*this time*" comment?
> Are you implying that you expect/suspect the results to change
> if the same test is repeated again (and/or many times)?

Yes. That is excatly what I would expect...

> Do you have any thoughts/assumptions as to the source of the
> non-repeatability (short of an explicit component failure)?

For example... MAP, current fuel/air ratio, air temperature,
spark energy, fuel temperature, the amount of leakage in the
fuel pump gears, etc...

> Jeff's test seems to show that the TCM fuel system works well
> (if setup correctly) and is even tolerant of potentially incorrectly
> applied high boost, (at least within the range of his test conditions).
> Jeff, can you pls share your test conditions i.e. density altitude,
> power settings, etc and re-confirm your normal fuel flow set points.

Assuming that Jeff's engine IS set up correctly between beers.
What has been shown by the present experiment is that at two sets
of MAP, temperatures, mixture settings, etc. that the engine tolerated
having a boost pump switched on. However, it experienced a large
excursion in fuel flow that could have been disasterous if the
fuel mixture was richer to begin with, the MAP was lower, or the
spark weaker.

> Bob, is your point that the TCM fuel system is poorly designed
> (or unsafe) because it doesn't work well if it is set up incorrectly
> (i.e. outside factory specs/range)?

No. My point is that the TCM fuel system design is faulty. It has
an extraordinary sensitivity to its input fuel pressure. That
problem can be made worse by bad setup, but even with "good" setup
it is still a bad design.

> You are of course free to fly behind whatever you like.
> But, throwing out innuendo without the associated facts,
> clarity, and/or remedies and/or implying that others who
> don't share your "feelings/opinions" are unsafe and/or
> uninformed actually confuses the point more than it helps.

I am not aware of the innuendo you mention.
The facts presented here in this forum by a number of TSIO-550
and IO-550 users support my conclusions.
Perhaps this message will help with the clarity that was lacking.

In previous messages, a few remedies have been offered.
The two most practical are:
1) regulate the pressure of the fuel just upstream of the injector servo,
2) use a Lycosaurus IO-540 fuel injection system (or the whole engine).

Actually I don't have a lot "feeling" on this matter, other than a
desire to keep my friends flying safely.  A number of people here
have pointed out that this system has quirks that can contribute to
unsafe flight conditions. A recent fatal accident was associated with
loss of power, black smoke from the exhaust and a question about the
boost pump switch located next to the gear switch. Sure training
and proper setup can reduce the risks, but not as much as elimination
of that failure mode.

I don't believe that I am confusing the point more than helping.
I'll be out on the porch having a beer with Jeff if you need me.

-bob mackey