That's a really interesting design philosophy being used by Cessna now.   In the Multiengine Cessnas (I put 3000+ hours in my C414A) the same setup was originally installed and it killed a few people.  Mandatory SB 88-3 required a change to LOW-OFF-HIGH switch by requiring the pilot to manually select HIGH and it was interlocked on the high side like the Columbia.    It turns out that the sensing switches and wiring were less reliable than the engine or mechanical fuel pumps they were supposed to protect. Glitches and failures in the sensing system triggered HIGH setting which combined with mechanical fuel pump pressure would kill an engine in the multis.  The bottom line was that manual selection of the high setting was deemed best practice by Cessna in 1988 in multiengine pistons.   In addition, HIGH would not sustain full power alone so MP had to be managed to meet fuel flow.  But, if HIGH was selected with the mechanical, it could produce very high flows and kill the engine so the MEB advised leaning to prevent such an occurrence.

Perhaps the sensing is now better or perhaps there has not been a repeat of the problems that led to changing out that design but the whole issue of auto HIGH fuel pump can really throw you for a loop if it comes into play at the wrong time and you haven't selected it manually.  It's worth thinking about how you would deal with an Auto HIGH kicking in properly or improperly.

Paul Miller
N357V Legacy
N700CS TBM700A

Richard T. Schaefer wrote:

Dan,

  My understanding (from reading the Columbia 400 manual) is that the Low mode is used for vapor suppression above 10,000 ft.

  The High is used when the mechanical pump fails, during prime, and prior to a hot start to cool any heat soaked components in the fuel system (as the hot excess fuel is returned to your fuel tank).

 

  When you setup your fuel system for your engine you should verify that high fuel pump mode will not flood your engine.

 

  In the Columbia (as I understand) the prime button engages the high fuel mode.

  They have a three way switch, LOW, OFF, and HIGH arm.

  The HIGH arm uses a fuel pressure switch and locking relay. The intent is if the fuel pressure drops while on HIGH arm, the HIGH mode of the fuel pump is engaged and is locked in, even as the fuel pressure rises as a result of the pump coming on.

   The HIGH arm is used during climb and cruise below 10,000 ft and set to OFF during descent. (And of course the LOW is used always above 10,000 ft).

   I assume you would go to HIGH arm on a go-around … one more thing to the checklist during a busy time.

 

   It’s important to make sure that HIGH and LOW are not engaged at the same time. So there is some care to how the primer button interacts with the three way switch.

 

  This is how I am setting up my IV-P.

 

r.t.s.

 

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From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Dan Reagan
Sent: Thursday, June 18, 2009 8:09 AM
To: lml@lancaironline.net
Subject: [LML] Boost pump question

 

Since my crankshaft bolt problem is causing me to change out my engine, I have decided to change from my current FADEC engine to a conventional mixture control engine.  The boost pump I currently have is a 2 speed Dukes and in the past was turned on and off by FADEC. (It could be manually turned on as when above 10,000 feet) 

 

My question is, when I am rewiring the fuel pump, do I need both the high and the low speeds?  My dumb question of the day is, when is the fuel pump supposed to be used on the TSIO550 in the Lancair IVP?

 

Dan Reagan

IVP 650 hours