X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 26 Sep 2007 08:22:17 -0400 Message-ID: X-Original-Return-Path: Received: from msgmmp-3.gci.net ([209.165.130.13] verified) by logan.com (CommuniGate Pro SMTP 5.1.12) with ESMTP id 2352119 for lml@lancaironline.net; Wed, 26 Sep 2007 01:38:21 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.165.130.13; envelope-from=ogrady@gci.net Received: from [10.0.1.8] ([24.237.13.108]) by msgmmp-1.gci.net (Sun Java System Messaging Server 6.2-3.03 (built Jun 27 2005)) with ESMTPA id <0JOY0018MNMDZ420@msgmmp-1.gci.net> for lml@lancaironline.net; Tue, 25 Sep 2007 21:37:32 -0800 (AKDT) X-Original-Date: Tue, 25 Sep 2007 21:37:20 -0800 From: Thomas O'Grady Subject: Re: [LML] Airspeed Indicator Consult In-reply-to: X-Original-To: Lancair Mailing List X-Original-Message-id: MIME-version: 1.0 X-Mailer: Apple Mail (2.752.2) Content-type: multipart/alternative; boundary="Boundary_(ID_4ycxb7oVz4OYJsvQE2z7Cw)" References: --Boundary_(ID_4ycxb7oVz4OYJsvQE2z7Cw) Content-type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Content-transfer-encoding: 7BIT I am guessing here, but is it possible that your pitot tube is leaking, but the leak was covered by whatever hose your mechanic connected to it? There is not much volume and hardly any flow, just the compressibility of the air, so it would take only a minute leak near the tip to bleed off pressure. Something as small as a tiny split in the first 1/16 inch or even a notch on the front of the tube might be enough. If the balloon covered the split, then the pressure would stay up. I agree that any opening downstream, like the gear speed switch would have let the pressure down quickly. Tom O'Grady On Sep 25, 2007, at 8:55 PM, Douglas W. Johnson, MD wrote: > In true medical fashion, I would like to request of you all 2nd and > 3rd opinions about the cause of an Airspeed Indicator (AS) problem > I developed last week. > > The problem: I am getting erroneously low (by 20-40kts) AS > readings. This came about suddenly, and has never been a problem > before. > > The history: > > First, you must know that the plane (L-IVP) has 970hrs TT and has > always had a very accurate AS, matching very closely GPS readings > (adjusting for wind and altitude) over the last 10 years. > > It is my habit to always verbally call out "airspeed alive" as I > accelerate down the runway for takeoff--the AS usually becomes > active at about 40kts or so. Recently, however, the AS did NOT > become active at the usual spot down the runway during a takeoff > run, despite a normal runup and full power settings. Just as I > pulled the power to abort the takeoff, the AI seemed to bump to > life. I exited the runway and regrouped. I shut down and inspected > everything externally--all looked good. Oh well, perhaps I had > inadvertently left the pitot cover on, and it later blew off as I > ran down the runway, I thought (no dice, as it turns out, as I > later found the cover safely stowed in the baggage compartment). > Sooo, I tried another takeoff with the same result--this time the > AS became active, but far down the runway, and I aborted again. > > The local A&P did a thorough inspection of the pitot system (all A- > OK), and did a full test of the AS with a meter attached to the > pitot tube showing that indicated AS was the same as the metered > setting on the calibration device through a full normal range of > speeds up to VNE. He also "set" an airspeed with the pressure on > the pitot and clamped it off, making sure there was no decay in the > indicated airspeed, which might have indicated a leak in the pitot > system. He also disconnected the pitot system within the cockpit > and reverse blew some air back out toward the pitot tube, on the > outside chance a bug or something had temporarily blocked the > system. All appeared normal, so I went out today to do another > test flight. > > Results: I ran down the runway today once again after normal runup > at full power. Once again, NO active AS at the usual point. I > decided to proceed with the takeoff referring to the GPS speed. > After takeoff and climbing into the pattern at 120kts (GPS) the AS > started swinging back and forth between 70 and 88kts--it never > stabilized and I could not even retract the gear, indicating that > the squat switch did not think I had enough airspeed to trip the > solenoid. I brought the plane around the pattern and landed without > incident, using the GPS speed indication + a good margin for > safety. The AS throughout the flight was always significantly below > normal reading by anywhere from 15-40kts. > > So, we have here a situation in which the AS tested out fine on the > ground, and bad in the air. Any idea why? > > The mechanic is stumped. I am stumped. One opinion has suggested > that the gear speed switch diaphragm may have ruptured, but > wouldn't that result in a decay in indicated speed with a test > device putting clamped air pressure on the pitot tube? > > Help and ideas appreciated. > Doug Johnson --Boundary_(ID_4ycxb7oVz4OYJsvQE2z7Cw) Content-type: text/html; charset=ISO-8859-1 Content-transfer-encoding: quoted-printable I am guessing here, but is it = possible that your pitot tube is leaking, but the leak was covered by = whatever hose your mechanic connected to it? There is not much volume = and hardly any flow, just the compressibility of the air, so it would = take only a minute leak near the tip to bleed off pressure.=A0 Something = as small as a tiny split in the first 1/16 inch or even a notch on the = front of the tube might be enough.=A0 If the balloon covered the split, = then the pressure would stay up.=A0 I agree that any opening downstream, = like the gear speed switch would have let the pressure down = quickly.

Tom = O'Grady

On Sep 25, 2007, at 8:55 PM, Douglas W. = Johnson, MD wrote:

=
In= true medical fashion, I would like to request of you all 2nd and 3rd = opinions about the cause of an Airspeed Indicator (AS) problem I = developed last week.
=A0
=
The problem: I am getting erroneously low = (by 20-40kts) AS readings. This came about suddenly, and has never been = a problem before.
=A0
=
The history:
=
=A0
First, you = must know that the plane (L-IVP) has 970hrs TT and has always had a very = accurate AS, matching very closely GPS readings (adjusting for wind and = altitude) over the last 10 years.
=A0
It is my = habit to always verbally call out "airspeed alive" as I accelerate down = the runway for takeoff--the AS usually becomes active at about 40kts or = so.=A0 Recently, however, the AS did NOT become active at the usual spot = down the runway during a takeoff run, despite a normal runup and full = power settings. Just as I pulled the power to=A0abort the takeoff, the = AI seemed to bump to life.=A0 I exited the runway and regrouped. I shut = down and inspected everything externally--all looked good. Oh well, = perhaps I had inadvertently left the pitot cover on, and it later blew = off as I ran down the runway, I thought=A0(no dice, as it turns out, as = I later found the cover safely stowed in the baggage compartment). Sooo, = I tried another takeoff with the same result--this time the AS became = active, but far down the runway, and I aborted = again.
=A0
The local = A&P did a thorough inspection of the pitot system (all A-OK), and = did a full test of the AS with a meter attached to the pitot tube = showing that indicated AS was the same as the metered setting on the = calibration device through a full normal range of speeds up to VNE. He = also "set" an airspeed with the pressure on the pitot and clamped it = off, making sure there was no decay in the indicated airspeed, which = might have indicated a leak in the pitot system. He also disconnected = the pitot system within the cockpit and reverse blew some air back out = toward the pitot tube, on the outside chance a bug or something had = temporarily blocked the system.=A0 All appeared normal, so I went out = today to do another test flight.
=A0
Results: I = ran down the runway today once again after normal runup at full power. = Once again, NO active AS at the usual point. I decided to proceed with = the takeoff referring to the GPS speed.=A0 After takeoff and climbing = into the pattern at 120kts (GPS) the AS started swinging back and forth = between 70 and 88kts--it never stabilized and I could not even retract = the gear, indicating that the squat switch did not think I had enough = airspeed to trip the solenoid. I brought the plane around the pattern = and landed without incident, using the GPS=A0speed indication=A0+ a good = margin for safety. The AS throughout the flight was always significantly = below normal reading by anywhere from 15-40kts.
=
=A0
So, we have = here a situation in which the AS tested out fine on the ground, and bad = in the air. Any idea why?
=A0
=
The mechanic is stumped. I am stumped. One = opinion has suggested that the gear speed switch diaphragm may have = ruptured, but wouldn't that result in a decay in=A0indicated=A0speed = with a test device putting clamped air pressure on the pitot = tube?=A0
=A0
Help and ideas = appreciated.

Doug = Johnson


= --Boundary_(ID_4ycxb7oVz4OYJsvQE2z7Cw)--