Gliders and bees

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James Kingdon
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Gliders and bees

Post by James Kingdon » Tue Jun 25, 2019 1:18 pm

No, not bee-strikes...

If you are flying a glider that is tied out, be aware that the pitot tube is a very inviting size for many solitary bees and parasitic wasps to make nests in. They typically put in some food, lay an egg and then seal the hole up nice and tight. The result is that an ASI that worked fine an hour ago may not work at all now, or if the blockage is only partial may behave in extremely unexpected ways with readings that are way off and typically change only slowly. Ideally gliders that are tied out should have the pitot plugged (and the plug marked with "remove before flight" ribbon), but wing tape is effective so long as we are careful to remove it during the DI. I would recommend that the ASI is tested before flight by gently blowing towards the pitot opening (non-contact) and check for needle deflection, but some people are very negative about that idea. Not sure why - explanations?

Finally, it's worth thinking about how you would handle a flight where the ASI was behaving strangely, it could well happen to you. I am not an instructor so it's not for me to say, but if you have any doubts it might be worth chatting with one and getting their opinions, or maybe take a flight with them with the instruments covered - it's been a while since a lot of us did that!

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Tony Firmin
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Re: Gliders and bees

Post by Tony Firmin » Tue Jun 25, 2019 2:24 pm

When a few of us owned a Puchacz that was tied down all summer, after taking the rudder off for some maintenance issue, we discovered swallows had built a nest underneath the tailplane, inside the fuselage.

Having flown the Grob 103 once without a functioning rear airspeed indicator I was pleased to discover how attuned my ears are to wind noise. I checked with the front pilot once or twice and I seemed to be good within about 5Kts. Clearly this is a glider dependent issue and may not translate to another type and I certainly wouldn't recommend putting it to the test.

Not blowing down the pitot is because you do not want any moisture to build up in the tube there is very little airflow when in use and a small blockage can make a big difference plus any breakfast crumbs that may follow your with your puff would be equally harmful. Corrosion from the moisture may also occur I'm guessing. Another blockage that I have experienced is the static ports on the rear of the fuselage quite possibly from an unthinking application of polish or wax to the gelcoat. Blockages do occur however and the tubes can usually be blown clear from the instrument end.

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Dennis Medland
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Re: Gliders and bees

Post by Dennis Medland » Tue Jun 25, 2019 6:57 pm

When we had our 123's I flew with a blocked pitot tube. With all the bouncing around on the ground, it's not possible to really be sure what, if anything is wrong. As one gains altitude, the ASI begins to act as if it were an altimeter; but not an altimeter that makes any sense. As Tony mentioned, your sensitivity to the wind noise will get you by along with the experience with the aircraft and how it "feels" in flight. One thing is certain; you're going to have to land the aircraft without the aid of your ASI.

It made sense to me at the time, to take the whole tow so that I could figure out how I was going to do the approach safely and to make sure I could "feel" my airspeed. Transport Canada says the ASI is a necessary instrument and you can't fly a plane that doesn't have a functioning ASI.........so land as soon as possible would seem to be the only option...........which I did safely.

After checking the tubing connections; which were all good, a small length of wire was advanced into the pitot tube. It didn't get far; maybe 5 cm. and then it hit the solid mud of the wasps nest. There was quite a lot of mud, which was blown free after breaking it up, from the plastic hose end i.e. inside to outside, so as not to damage the ASI.

All in all, a white knuckle experience I hope never to repeat; although I do take comfort in knowing that I did it once successfully.

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Tim Wood
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Re: Gliders and bees

Post by Tim Wood » Tue Jun 25, 2019 8:58 pm

Blowing down the pitot has a far more serious potential effect than moisture. It can ruin your airspeed indicator.
Been there

Tim

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James Kingdon
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Re: Gliders and bees

Post by James Kingdon » Tue Jun 25, 2019 9:00 pm

Um, how? The sole purpose of the pitot is to receive positive air pressure. Now clamping your lips around it and seeing if you can beat the pressure record could be a bad idea, but blowing towards it?

Alex Upchurch
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Re: Gliders and bees

Post by Alex Upchurch » Wed Jun 26, 2019 8:26 am

Um, how? The sole purpose of the pitot is to receive positive air pressure. Now clamping your lips around it and seeing if you can beat the pressure record could be a bad idea, but blowing towards it?
How hard are you planning to blow? Because as I recall, 1.5 psi works out to about 250 Knots. Most ASI don't like going vastly over-range.

Alex

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Ivor David
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Re: Gliders and bees

Post by Ivor David » Wed Jun 26, 2019 11:13 am

It amazes me how little we see this problem considering that our pitots are not normally covered when the aircraft are parked. Hence the need to stress that airspeed is controlled by the glider's attitude w.r.t the horizon and verified by the level of wind noise. In higher performance gliders the wind noise is generally less but is still audible and needs to be listened to.

The obvious answer is to ensure that the piiot is covered when the aircraft is parked for a period - which then brings on the responsibility to ensure the cover is removed before flight. Powered aircraft pitot covers are easily seen during a walk-around although I don't think these covers would suit our gliders

It a good job that we don't fly at night or in IFR conditions where lack of an ASI can have serious consequences. Many lives have been lost in airline accidents caused by a tiny little bug setting up house in a convenient pitot tube.
Ivor David

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James Kingdon
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Re: Gliders and bees

Post by James Kingdon » Wed Jun 26, 2019 11:21 am

Alex Upchurch wrote:
Wed Jun 26, 2019 8:26 am
How hard are you planning to blow? Because as I recall, 1.5 psi works out to about 250 Knots. Most ASI don't like going vastly over-range.

Alex
From a distance of 3 or 4 inches I blow as if putting out a candle. I can't usually see the result, but I'm told it gives an obvious twitch of a few tens of knots. It would be interesting to test in a controlled environment, but I wouldn't have thought it possible to blow the 150+knots it would take to max most asi. But each to their own.

Biggles
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Re: Gliders and bees

Post by Biggles » Wed Jun 26, 2019 11:39 am

During DI on T21, I just tap the end of the pitot tube with the ball of a finger tip and that causes the twitch of the needle, no need to blow.

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