Primarily for general aviation discussion, but other aviation topics are also welcome.
#1810102
Given the discussion on Airspace infringements I though the information below (from a post on Gliderpilot.net) might be of interest - especially anyone operating a TMG in a non-gliding environment where the maintenance team might not be so familiar with "Total Energy" probes and their plumbing. Note that it would also be easy to connect the TE line to the Altimeter, with similar errors - the correct connection is of course TE probe to variometer, static to Altimeter/ASI/Transponder.

Something from the latest CHIRP to be aware of:
https://members.gliding.co.uk/2020/11/19/latest-chirp-report/

Report No.4 – Transponder Accuracy
Report Text: I have a hand in maintaining a club motor glider which had a transponder (XPDR) issue; the aircraft in question has a ModeS XPDR. Various pilots reported multiple ATC requests to confirm altitude, with one in particular needing to prove that he had not infringed controlled airspace. There appeared to be a discrepancy of approximately 400ft. I took a flight with one of the reporting pilots and we checked readings with ATC, which appeared to be within limits. Some head-scratching followed as this result did not confirm reports. The issue was subsequently confirmed to me when asking for a transit clearance; the associated check of altitude and pressure setting confirmed approximately 400ft excess altitude being transmitted by the ModeS XPDR in ACS mode with extended squitter - I put the XPDR U/S. I then took to the books and tried to figure out what could be amiss. Eventually it became clear – as a motor-glider, as well as normal Pitot and Static pneumatic connections there is also a Total Energy (T/E) connection. I did the sums for the pressure reduction provided by T/E at the cruise airspeed and this related to about 400 feet! A quick look behind the panel revealed that there were 2 (unmarked) black pneumatic tubes and - yes you have guessed – the T/E was connected to the XPDR and the Static to the variometer (a sensitive rate of climb instrument - T/E is used to remove speed variation indications (stick lift) in gliding so that you only ‘read’ energy gain (or loss)). Problem solved! The tubes are now labelled and will, in due course, be replaced with new colour coded (and labelled) replacements! Unmarked connections of the same type and colour with different functions is not good engineering practice!
#1810133
We used to have radio annuals to stop this as a part of a certified electronic conspicuity system (see my other forum thread on the late and great Richard Trim OBE). However, I believe that BGA, BMAA and LAA aircraft don’t have that requirement? Even for EASA ELA 1 aircraft, the the following is still required according to AMC M.A.301(i):

Radio and electronic equipment: Inspect for improper installation and insecure mounting. Carry out ground function check.
Pitot-static system: Perform operational check
Transponder: Perform operational check

So I believe that could be an air check of “Brize Radar, G-XXXX, can you see my transponder in the vicinity of Little Rissington”. The problem with airborne checks is that there are errors to them - the reported level on the Mode C by the ATCRU could be say FL020, but the picture in 6-10 seconds of lag, the encoder could be 50-100ft out, and then the altimeter it is being checked against the best part of another 50-100ft out. So the bench test at the radio annual was the way to ensure that the output of the transponder was certified as correct. As we go for more carry on equipment for electronic conspicuity (SkyEcho, FLARM, PAW, etc...), plus the use of official/unofficial ground stations retransmitting that information, plus the loss of radio annuals, then the safety certification of that electronic conspicuity system is ever harder to assure. Which may be fine for VMC/VFR, but how many of these untested and uncertified systems are being used for IFR or IMC?
#1810134
Genghis the Engineer wrote:Interesting, I'd only comment that those issues could arguably have occurred on any class of aircraft. That it occured on a motorglider is essentially happenstance.

G


I think the subtlety is that the TE / Static mix up means the system appears to function normally, errors are small and increase/decrease with airspeed, so it would'nt necessarily be obvious on a test flight (this one had presumably been like this for some time). Also TE probe normally tail or fuselage mounted, so both static and TE lines disappear back into the fuselage. On gliders the factory installed lines are always colour coded (AFAIK), but on a TMG it could be that the TE was installed as an add-on later?

As you say this is one of a generic class of pneumatic miss-connection problems, but some of them tend to show up a bit more clearly. If a TMG is being maintained under (say) LAA I wondered how many LAA inspectors would be familiar with TE Probes and their plumbing? Even in this case it clearly took the inspector (BGA rated?) a while to diagnose the problem. A bench test might not have shown it, even in one the panel if you tapped into what you thought was the "static" line near the transponder......

Hopefully it will be a one off.... anyway, just for interest.....
Flyin'Dutch', Ben K, AlanC and 4 others liked this
#1810182
jrp wrote:For the avoidance of doubt would it help those reading this thread to be given a quick explanation of TE probes and why they are a gliding thing.


If a glider is flying at, say, 80 kt and pulls up into a thermal, the variometer will show a huge rate of climb. Eventually it will settle down to the actual rate in the thermal.

But the glider pilot wants to know almost immediately how strong the thermal is. Stopping for weak thermals slows your XC speed.

A total energy system fixes this by inserting a calibrated leak into the system. This cancels out variometer changes caused by changing airspeed, so the pilot always sees the true rate of air movement.
Flyin'Dutch', gaznav liked this
#1810195
It simply but cleverly tells the gliding pilot what the change in total energy (potential and kinetic energy) is.
#1810214
Total Energy compensation in practice.

How it works: The variometer

From the latter link:
AOPA wrote:Some total energy systems use a small probe mounted in undisturbed airflow, often ahead of the tail fin, that is connected to the static outlet of the variometer. When airspeed decreases in a climb, suction from the probe decreases and offsets the pressure at the static outlet of the variometer; in a descent, increasing airspeed increases suction. You’re left with an indication of the movement of the air mass around you.



Powered aircraft (other than TMGs & SLMGs) don't have them because their pilots are typically not interested in immediate information about thermals and sink since they get their means of propulsion and lift from the fuel tank rather than the atmosphere.
gaznav liked this
#1810216
https://www.aopa.org/news-and-media/all ... variometer

Glider pilots need to know what the air is doing. Their varios tell them that, not whether the aircraft is climbing or descending. Total energy means the sum of kinetic and potential energy - this increases when the glider is in a thermal.
Powered aircraft increase their total energy by the power delivered by the engine so total energy compensation is meaningless.
(A physics nerd would say that the powered aircraft is actually trading chemical energy in the fuel for kinetic and potential energy)

Edit: blast, beaten to it!
gaznav liked this
#1810218
Potential energy is the amount of energy stored in an object due to its position.

A brick on earth has 0 potential energy; a brick held up in your hand at 6 feet high has potential energy - you let go, it will fall down.

Kinetic energy is the energy an object has based on its movement.

That brick starts off with 6 ft worth of potential energy but it has zero kinetic energy. When you let go that potential energy will go to zero by the time it will reach the earth the potential energy but it has traded that for the kinetic energy or speed with which it will end up.

An aeroplane has both, you will no doubt have noticed that if you are flying along and pull the stick back that the kinetic (speed) energy will decrease and the potential energy (height) increases.

The only way you can increase potential energy by not losing kinetic energy is by sticking more energy in the system by opening up the throttle and ensuring that the kinetic energy is going to stay the same in the climb.

(Most) gliders don't have an engine but piggy back onto rising air, the thermals, basically lifts in which you circle to go up.

In the good old days people did have VSIs in gliders which would tell them they would go up (the reduction in atmospheric pressure is measured by the VSI as air flows through the thing - tell me if you need a refresher on how VSIs work) when the glider/aeroplane goes up. And as there is no measuring the reduction or increase in speed in a classic VSI you don't know if you are just going up (but trading speed) or whether you are actually gaining energy (as you would if you open the throttle)

So someone clever thought out that if you develop a clever system that compensates for the variation in kinetic energy due to speed changes you will get a useful tool to see if your circling is indeed giving you additional energy or whether you are just having a climb indication from pulling on the stick.

Therefore a compensated (for speed changes) VSI measures both differences and height differences and so becomes an energy meter - that is represented as on a VSI scale.

Why do aeroplanes not have them ?

1. Due to the slipstream of the propellor it would be difficult to calibrate one as they have to be out of the slipstream as prop wash speeds vary with engine settings and not per se with airspeed changes (gliders with engines therefore often have a seperate setting for the VSI to sort this out when operating with the engine)
2. Speed changes in aeroplanes are less abrupt as they are in gliders when the latter are thermaling, powered aeroplanes bring their own thermal to the equation.

Hope that helps
gaznav liked this
#1810229
Whenever there's a discussion here that involves gliding it makes me realize that I must do some again.
I once lost a student because I sent him for a day to the local gliding club so he could get an understanding of soaring and also do some spinning - in one day he was hooked and didn't come back to powered flying. I learned (something) about flying from that ...!
Flyin'Dutch', ls8pilot liked this
#1810234
Some good explanations here, just to add and explain how it is done why the plumbing matters.....

The most common system used in glider/TMG systems is to have a circular probe perpendicular to the airstream with two holes or slots at the rear (ie opposite to the airlow). Due to the venturi effect from the flow of air round the probe you get a pressure which is the negative equivalent of what a pitot probe gives, so a small suction. The faster you go, the bigger the suction (in fact you can make an ASI work by plumbing T/E to Static and Static to Pitot at the instrument - the pressure difference is the same but provided by sucking not blowing). So TE = - Ptot

TE probes come in any shapes and sizes and typically project from the fin or stick up from the fuselage aft of the wing,

Image


You then take a Variometer (which is a sensitive VSI) and substitute this negative TE pressure for static. As a Variometer/VSI measures change (in a mechanical system flow into or out of a closed capacity) then at steady speed it makes no difference that the "static" is in fact lower (-Ptot). But as you slow up the -Ptot "TE suction" will decrease, so (in still air) allowing air to flow into the capacity and cause an indication of descent. Due to some rather complex maths ,and very conveniently, the transient descent indication is balanced by the transient climb the glider does while slowing up (trading speed for height).

So the impact is that in still air (ie air that is not rising or descending) you can zoom up several hundred feet from say 100kt to 50kt and the Variometer will not show a huge climb - merely slowly go from your sink rate at 100kt (say -400fpm) to that at 50kt (say -150fpm).

The opposite happens as you accelerate...

One weird effect you get with a glider is that in the ground run the Vario will show "Climb" well before you leave the ground, this is because the TE "suction" is increasing as you accelerate, pulling air out of the capacity. In fact the glider is getting energy input into the system from 180hp+ donk in the front of the tug, which the Vario can only show as "climb".

So TE is not a "leak", it is negative Pitot. There is a calibrated leak, called Netto, that can be used to factor out the natural sink of the glider - so that in still air the vario shows zero, whatever your airspeed and resultant sink rate, but that is another topic!

Hope this helps!

*edited to correct some errors !
MikeW liked this
#1810236
The total energy systems certainly add another potential source of error. My experience of 'ordinary' systems has been pretty mixed.

Altitude encoder drifted to around -350ft - fairly easy to recalibrate with the aid of a vacuum pump and calibrated gauge. Altimeter and encoder error of almost 400ft - easy repeat the encoder calibration - only problem - it was the altimeter which had drifted and I could not recalibrate it! So I currently have a nice (expensive!) altimeter in the panel....

Nothing last forever and a routine check of encoder and altimeter is a prudent precaution paticularly given the NATS CAIT tool!