X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 11 May 2008 20:01:33 -0400 Message-ID: X-Original-Return-Path: Received: from mta13.adelphia.net ([68.168.78.44] verified) by logan.com (CommuniGate Pro SMTP 5.2.3) with ESMTP id 2916366 for lml@lancaironline.net; Sat, 10 May 2008 09:44:11 -0400 Received-SPF: pass receiver=logan.com; client-ip=68.168.78.44; envelope-from=glcasey@adelphia.net Received: from [76.89.163.133] by mta13.adelphia.net (InterMail vM.6.01.05.02 201-2131-123-102-20050715) with ESMTP id <20080510134239.PRG21875.mta13.adelphia.net@[76.89.163.133]> for ; Sat, 10 May 2008 09:42:39 -0400 Mime-Version: 1.0 (Apple Message framework v753) In-Reply-To: References: Content-Type: multipart/alternative; boundary=Apple-Mail-73-191576896 X-Original-Message-Id: From: Gary Casey Subject: Re: Lancair 320 low fuel pressure X-Original-Date: Sat, 10 May 2008 06:43:30 -0700 X-Original-To: "Lancair Mailing List" X-Mailer: Apple Mail (2.753) --Apple-Mail-73-191576896 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed I've got another idea that might relate to why Cherokees, RV's and your 320 randomly exhibit the same phenomena. The key element is that for a given plane the behavior is consistent. I suspect you are reading the pressure with a conventional mechanical gage, like in my old Cherokee, with an "old fashioned" can-type sending unit. If not, you can hit the delete button now. Most mechanical gages are vented through miscellaneous leaks in the case - there is no dedicated vent hole that allows the measured pressure to be referenced to atmosphere. What would happen if the case was inadvertently sealed? The interior would probably be at approximately seal level pressure, starting at the pressure in the factory in which it was built and then ever-so-gradually assuming the pressure at which the aircraft is stored, possibly taking anywhere from days to months get there. When you go to altitude the absolute fuel pressure will drop, since the fuel pump produces pressure relative to the existing atmospheric pressure. But the gage would be(inadvertently) measuring the pressure relative to sea level. The pressure difference between sea level and 8,000 feet is between 3 and 4 psi (to be precise the standard pressure at 8202 ft is 3.9 psi less than sea level). That could explain the difference. And the real (what we call gage pressure) might a little higher at the higher elevation because the flow rate is a little lower. At the slight risk of damaging the pressure gage you could drill a tiny hole in the housing at the bottom in order to provide a positive vent. It's probably not a fancy gage that can be disassembled easily, so you almost have to just take a chance. Gary Casey > > For years I flew a Cherokee 180 that had a similar problem. When > going high, above 8,000 feet and especially when the outside > temperature was warm, the fuel pressure would gradually drop to > about 2 psi. At low altitude it would be in the high end of the > green and at high altitude the low end of the green. I could never > come up with an explanation and the condition didn't change with a > new fuel pump. It would rise slightly when turning on the electric > pump. I doubt that it is a tank vent problem, but I have no likely > explanations. Never caused a problem, but always bothered me. I > always wondered if it was something to do with the way the gage > read the pressure, but that didn't make sense either. > Gary Casey >> >> I normally have had around 4-6 psi flying around my airport, however, >> yesterday, I flew on my first cross country. Cruising at 220mph, I >> noticed my >> fuel pressure slowly start to drop as I got faster and higher. It >> went down >> to 2 psi. I climbed higher in case I needed time to find a place >> to land. >> Luckily I didn't. >> >> Matt Reeves --Apple-Mail-73-191576896 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1 I've got another idea that might relate to why Cherokees, RV's and your = 320 randomly exhibit the same phenomena. =A0The key element is that for = a given plane the behavior is consistent. =A0I suspect you are reading = the pressure with a conventional mechanical gage, like in my old = Cherokee, with an "old fashioned" can-type sending unit. =A0If not, you = can hit the delete button now. =A0Most mechanical gages are vented = through miscellaneous leaks in the case - there is no dedicated vent = hole that allows the measured pressure to be referenced to atmosphere. = =A0What would happen if the case was inadvertently sealed? =A0The = interior would probably be at approximately seal level pressure, = starting at the pressure in the factory in which it was built and then = ever-so-gradually assuming the pressure at which the aircraft is stored, = possibly taking anywhere from days to months get there. =A0When you go = to altitude the absolute fuel pressure will drop, since the fuel pump = produces pressure relative to the existing atmospheric pressure. =A0But = the gage would be(inadvertently) measuring the pressure relative to sea = level. =A0The pressure difference between sea level and 8,000 feet is = between 3 and 4 psi (to be precise the standard pressure at 8202 ft is = 3.9 psi less than sea level). =A0That could explain the difference. =A0And= the real (what we call gage pressure) might a little higher at the = higher elevation because the flow rate is a little lower. =A0At the = slight risk of damaging the pressure gage you could drill a tiny hole in = the housing at the bottom in order to provide a positive vent. =A0It's = probably not a fancy gage that can be disassembled easily, so you almost = have to just take a chance.

Gary Casey


For years I flew a Cherokee 180 that had = a similar problem. =A0When going high, above 8,000 feet and especially = when the outside temperature was warm, the fuel pressure would gradually = drop to about 2 psi. =A0At low altitude it would be in the high end of = the green and at high altitude the low end of the green. =A0I could = never come up with an explanation and the condition didn't change with a = new fuel pump. =A0It would rise slightly when turning on the electric = pump. =A0I doubt that it is a tank vent problem, but I have no likely = explanations. =A0Never caused a problem, but always bothered me. =A0I = always wondered if it was something to do with the way the gage read the = pressure, but that didn't make sense either.
Gary = Casey

I normally have had = around 4-6 psi flying around my airport, however,
yesterday, I flew = on my first cross country. Cruising at 220mph, I noticed my
fuel = pressure slowly start to drop as I got faster and higher. It went down =
to 2 psi. I climbed higher in case I needed time to find a place to = land.
Luckily I didn't.

Matt = Reeves

= --Apple-Mail-73-191576896--