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..did I turn the wrong way? Again!

Posted by on in Training
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I heard a few post flight conversations lately - that's why I am writing this:

Did this happen to you recently or regularly?
You fly along between thermals and all off a sudden your variometer goes ooorrff.
"Beaut" you think and you start to turn, only to find that you turned into sink. And not just a little bit of sink. No, a whole lot of going doooown - you just lost 150 feet in that turn!

"Sink makes you think" and maybe doubt your ability, but: "You can't be that wrong all the time!"
OK, for starters you should fly by feel and not just by what your variometer tells you. Varios like to tell lies at times - if you let them.

My remedy for "instrument pilots" is to blind off instruments on the panel in the front seat of a two seater, turn the variometer sound down and "make" you listen to the sound of the airspeed and feel the air and sensations of micro g-forces instead of just being an IFR pilot.
The "snaking" S-bend manoeuvre instead of flying a whole turn is also highly advisable to avoid reducing your xcountry speed unnecessarily.

But back to the variometer system:
The total energy compensation might be "out" in your glider. This might be very likely if you find that you did not feel any lift but your variometer is still going ooorrrff on a regular basis. Hint: there is no thermal if your altimeter does not go up after you established thermal speed.

There are certain atmospheric conditions that create a lot of horizontal gusts and lure you into turning when you really shouldn't.
The effects of horizontal gusts on a variometer system with incorrect compensation will be magnified and result in a lot of false readings.

Type "horizontal gusts variometer" into Google and you will find pages and pages of information on the influence of gusts on variometer systems.
Electronic accelerometers and specialised algorithms were implemented in electronic variometers as a result  of research findings.
The research happened about 6-7 years ago in the "Akaflieg" (groups of aerodynamical engineering students from individual German Technical Universities).
This should be the end of phantom lift! Not quite.
New varios such as the LXNav or ClearNav systems are using accelerometers and algorithms for the last 2-3 years to compensate the effects of gusts and reduce the resulting false vario indications. The hardware accelerometers were in the instrument since they were built. New firmware did activate and made use of the accelerometer data more recently.
However, these newer variometer systems still require proper compensation. Just as "older" systems like the Cambridge or Borgelt system or even the mechanical Winter / Sage / Boli variometer.
Newer variometer systems are able to be entirely electronically compensated. In this case the variometer plumbs only into the static and total pressure system - no TE-Probe required. The computer does the rest, end of story - you might think. Not quite!
It is always imperative to have a fault free static and total pressure system. No leaks, kinks, sharp bends, dust, oil, water or insects in the system!
Pressure ports have to be in the correct location as well. The computer will do all the compensation as programmed (adjusted). But remember: garbage in - garbage out! A computer can't rectify problems that are caused by hardware that is not working correctly.
The preferred method of total energy compensation is still by the means of a "TE-Probe" - no matter if the system is old or new.
If your glider has got a "TE-Probe" or is entirely electronically compensated - you can check its function by following the steps below.
Esa-systems also provides info about "probe manipulation". But: filing off or extending, widening or reducing should only be done as a last resort after making sure that the pneumatic plumbing of the glider is fault free!
(Taken from http://www.esa-systems.com/index_en.html - "ze Germans did it again..")
In a nutshell: the variometer should never give a positive reading during a pull up. If it does - check the plumbing system first! Might be as simple as: do you really need six connectors / joiners / T or Y pieces between the port and instrument?
Mr Borgelt did a write up as well - but Esa's is more concise and included a neat diagram! (a picture is worth a thousand words)
For a reliable check, it is necessary to conduct the flights in absolutely calm air (first sufficient light prior to or at sunrise, no wind). Check flights do require some experience, and the instrument readings should be, if feasible, recorded by a video camera. This would help to eliminate interpretations influenced by emotions . Normally esa-systems probes and venturi tubes are perfectly compensated and do not need any readjustment. In exceptional cases of over- or undercompensation, depending on the particular glider, adapting the probe may be necessary. The sketch will help to interpret the variometer readout correctly.



  • Les Milne
    Les Milne Wednesday, 04 February 2015

    And lets not forget that however much time & effort you put into your instruments, including the above, you are still reading history.

  • Erich Wittstock
    Erich Wittstock Wednesday, 04 February 2015

    Would you like the truth or a lie from history?

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