Gertrude askedHere are some considerations from the tow pilot's point of view. It happened to me on my first flight of the day at the Canadian Nationals a few years ago. I noticed during my RPM and airspeed check before lift off, that the ASI needle was bouncing around zero. Since I had an open-class glider behind me with water ballast, I made a quick decision to continue the flight, rather than have the glider run into me on the runway as I stopped. I continued the climb, using attitude for speed control and by the time I reached release height, the needle had come up to the correct speed.

We had had a big rainstorm the previous evening and water must have entered the pitot tube, which on a Bellanca Scout, angles slightly upward on the ground. During the flight, the dynamic pressure must have slowly displaced the water into a water trap in the system.

Ironically, each previous day, I had made a quick circuit before towing. For some reason on this day, I didn't!

Here's a picture of the pitot/static system, with a different reason for ASI failure. Unlucky bee!

http://i30.photobucket.com/albums/c3...npitottube.jpg

I had virtually the same problem as India Four Two a while back, towing a Skylark 4 glider out of Nympsfield (Bristol Gliding Club) with a Jackaroo (4 seat Tiger Moth conversion, for those too young to remember) at a competition.

At about 100 ft over the edge of the airfield, crossing the ridge, a significant bit of turbulence must have displaced some trapped water, asi flickered and dropped to zero. It would have been unsociable to dump the glider, and unnecessary - I carried on with the tow (and subsequent circuit) on attitude and power, and parked while disconnecting the pitot and static lines and blowing out the water.

I declined the offer to carry on towing before fixing the problem, and the competition director quite understood my point of view.

ASI failure
 

Remember the approximate attitudes for power/ flap configuration and you will not be too far out from normal parameters.

A tip for the Cessna 150/152/172 aircraft, when on the approach to land ensure that the ventilator tube into the cockpit from the wing is opened. You can hear the change in sound as the centre of pressure moves forward as the angle of attack increases. It used to be a good way to demonstrate a precautionary approach with a covered or failed Air Speed Indicator.

something I didn't mention in the original post
 

The go - no go decision is exactly the same for any takeoff, glider, tug, light aircraft or Boeing. It has to be made at the most awkward moment!

Thank you for all your comments. I've been thinking about my experience while driving back and forth from the airfield, and realise now that I was being economical with an important detail of PILOT failure on this flight without a serviceable ASI.

The overloaded mind effect.

I do a lot, and I mean a lot of my flying in the back seat of a training glider, the good old Kl3.

On this occasion, rigged the Pegasus single seater which I had flown only 4 times before this season, and in a hurry to get launched. So overlooked the white tape on the pitot tube. Correct decision not to abort. For all kinds of reasons. Thinking about best plan to do after releasing. Followed tug to cloud base. And absolutely BLANKED on what knob to pull to release from the tug. That actually was a moment of panic. Being in a different glider from my customary steed, it of course was not in the usual location. After a moment of complete blank, I said to myself "LOOK AT THE KNOB BEFORE YOU PULL IT, STUPID WOMAN!" At which point the British Gliding Association requirement came to my rescue. All release knobs in gliders are painted YELLOW. I located the correct one to pull and pulled. So this story ended happily.

But my moment of panic was forgotten in the subsequent management of the flight, and I only retrieved it to mull over on the drive home, and now share it with you, my electronic friends!

Mary

So that's what causes the wheeze in the flare... often wondered!

:ok:Power + attitude = performance. Know your aircraft.

CFI suggested leaving a glider out overnight last week - he taped up all the statics. He also put a single line of tape over the ASI and altimeter, seemed like a good idea and would be an unmissable hint when you're preparing to fly.

They do, to avoid the kind of situation shown in these videos:
<http://www.gliding.co.uk/bgainfo/safety/winch-safety.htm>

In addition, typically before flying solo, a student glider pilot will have had to demonstrate that they can fly an entire flight with all their instruments covered up.

At my club it's a circuit with the altimeter covered up, not all the instruments.

Can't remember if I ever did it with both covered simultaneously, but I distinctly remember having to demonstrate several circuits with each of the altimeter and the ASI covered.

However, I learned something of relevance a couple of weekends ago whilst doing my tailwheel checkout on a decathlon. This particular machine was devoid of many avionics, including no stall warner. My accomplice for the training took us up to 4000ft and we did some stalls and made me take note of the stick position. It's an extension of the trim principle, and quite obvious when you think about it. Stick position sets airspeed. In unbanked, unaccelerated (i.e. 1G) flight, if you don't change the configuration (flaps, load etc), the aircraft will stall at a constant stick position. If the stick's forward of that position, it won't stall. End of story.

Now a decathlon doesn't have flaps - so we made a lot of approaches in some pretty leery sideslips where the ASI doesn't necessarily read quite right. The whole stick position thing worked rather nicely.

Just another tool..

A few days ago we ran an airplane around the pattern following some major work. Several squawks were observed...including no airspeed. An obstruction, most likely an insect, was discovered in the pitot system.

Twice I've experienced pitot blockages in large airplanes, when the culprit was dead insects that swelled with contact with airborne moisture. Another occasion involved a considerable amount of water in the static system, courtesy of very large WWII sized static ports and a stiff, horizontal rain. It happens.

A variety of means of handling the problem can be had. Pitch and power works great...if you've got power. If not, then the power, not so much. You still have the sound of airflow over the cockpit. You have the feel of the controls. You have the angle the wing makes with the horizon, as well as indicated pitch (if you have an attitude indicator). You may find that your aircraft settles to a desired approach speed when you simply set the trim to a known setting, and relax the controls...trim pitch to fly the desired speed, and make slight adjustments. Approach a little high in the glider, slip as needed.

The day I received my new temporary flight instructor certificate, I climbed into my rented airplane to head home...and observed a tennis ball fitted neatly over my pitot tube...after takeoff. Here I was, a newly minted flight instructor, of all things, in the air with a tennis ball over my pitot tube. I flew home, parked not far from the flying club, and removed the tennis ball (pitot tube cover).

It happens. When it does, see it as a learning experience.