Wobbly altimeter ...
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Wobbly altimeter ...
Hi guys, one of my (steam driven) altimeters has just developed a wobbly "hundreds" hand : it oscillates about a hundered feet either side of the reading it should be giving at any given time (i.e. if I'm at 2,000 feet it will vary between 1,900 and 2,100 feet constantly).
Obviously there's a problem somewhere : I have a second altimeter which does not display this problem, which would obviously indicate a knackered first altimeter.
However, I've just today noticed the VSI doing something very similar for a short period of time. Is there potentially a common issue, or something more serious than a duff altimeter ?
Obviously there's a problem somewhere : I have a second altimeter which does not display this problem, which would obviously indicate a knackered first altimeter.
However, I've just today noticed the VSI doing something very similar for a short period of time. Is there potentially a common issue, or something more serious than a duff altimeter ?
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My #1 altimeter does this too, by about 20-30ft either side, but only rarely.
It would suprise me if it was water ingress since the pipe between #1 and #2 altimeters is very short.
Isn't there some sort of damping mechanism?
It would suprise me if it was water ingress since the pipe between #1 and #2 altimeters is very short.
Isn't there some sort of damping mechanism?
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Yes, one is an encoding altimeter which drives the altitude capture feature of the KFC225 autopilot. Otherwise the two appear identical. Could the encoder cause this?
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Now that's interesting ... I've also been having problems with spurious mode C transmissions from the transponder. So could a duff encoder cause all these things ?
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As a pilot not an aircraft engineer.
On the encoding ones they have a vibrating thingy which is meant to reduce the lag in it. I suspect there is something up with that.
Reason.
If you had a leak in the static system the error would be pretty constant for a set attitude. And it would produce an error in your airspeed as well.
If it was water it would again give you a constant error.
You have 2 different types of Altimeter and we now have 2 aircraft without a fault on the basic altimeter but have one on the encoder.
The encoder does have a vibrator in it which by very nature is cyclic, you have a cyclic error.
Added in as well the mode C issues
Now the VSI giving you grief as well I haven't quite fitted in with my theory. But I suppose that what ever is driving the encoder is actually moving the bellows in it and I suppose it could be sending a back pressure down the tube. But with the lag in the VSI I wouldn't have expected it to register such a cyclic disturbance.
Personally as a none engineer I would get an avionics licensed type person to disconnect all the instruments on the static system and then block off all bar the last on the line and blow low pressure compressed air back through it (don't use an oxygen bottle it will have way to high a pressure on it). Then repeat for every outlet then go back to the first one again for a final squirt. If that doesn't fix it send your encoder off for service.
Or you could say sod it the Altimeter is a primary important bit of kit, its been in there for 15 years with nobody touching it. A service is small money in the grand scale of things and just do that first anyway.
On the encoding ones they have a vibrating thingy which is meant to reduce the lag in it. I suspect there is something up with that.
Reason.
If you had a leak in the static system the error would be pretty constant for a set attitude. And it would produce an error in your airspeed as well.
If it was water it would again give you a constant error.
You have 2 different types of Altimeter and we now have 2 aircraft without a fault on the basic altimeter but have one on the encoder.
The encoder does have a vibrator in it which by very nature is cyclic, you have a cyclic error.
Added in as well the mode C issues
Now the VSI giving you grief as well I haven't quite fitted in with my theory. But I suppose that what ever is driving the encoder is actually moving the bellows in it and I suppose it could be sending a back pressure down the tube. But with the lag in the VSI I wouldn't have expected it to register such a cyclic disturbance.
Personally as a none engineer I would get an avionics licensed type person to disconnect all the instruments on the static system and then block off all bar the last on the line and blow low pressure compressed air back through it (don't use an oxygen bottle it will have way to high a pressure on it). Then repeat for every outlet then go back to the first one again for a final squirt. If that doesn't fix it send your encoder off for service.
Or you could say sod it the Altimeter is a primary important bit of kit, its been in there for 15 years with nobody touching it. A service is small money in the grand scale of things and just do that first anyway.
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I thought all "sensitive altimeters" use vibration to eliminate stiction - regardless of whether they contain an encoder or not.
It is probably highly product specific...
It is probably highly product specific...
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Your normal altimeter fitted to most training aircraft isn't a sensitive one. These are the same as most standby altimeters. Which is why its not abnormal to have anything up to 500ft split in them depending how fast you are climbing. They will sort themsleves out with a wee bit of a split in cruise. They don't use sensitive ones on the standbys because they are more expensive and require a power supply.
The encoder bit is just an add on to the sensitive altimeter.
There used to be tons of errors and encoder issues (not with standing human error with setting the thing) which is why most transponders now have there own built in pressure sensor.
As I qualified I am a meer driver with a theory not a avionics engineer
The encoder bit is just an add on to the sensitive altimeter.
There used to be tons of errors and encoder issues (not with standing human error with setting the thing) which is why most transponders now have there own built in pressure sensor.
As I qualified I am a meer driver with a theory not a avionics engineer
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They don't use sensitive ones on the standbys because they are more expensive and require a power supply.
which does indeed require power because the digital portion is servo-driven. (I have read somewhere that somebody has managed to make one of these which doesn't need power, however).
I am 99% sure my "sensitive altimeter" (here) does not have a power connection, except for the light.
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I seem to remember there are some rules and regs about what height the service ceiling is and the requirement for a sensitive alitmeter.
The vibrating thingy requires power and also so does the encoder electronics bit. And of course your light which to be honest is proberly on a seperate circuit being powered by your flight director alt select logic.
If you go into the back of them a sensitive alt has a din plug fitting and static in the back of them and the unsensitive ones just have static input.
We used have both types in pieces in the flying school and the sentive one/encoder has a whole heap of gubbins/wires that the none sensitive ones don't have. The old ones had a wee out of balance motor and the new ones have a pizo electric vibrator.
And to add the standby alt in me fancy EFIS turboprop isn't a sensitive one and only has one static input from the 3rd static port. All the others on the aircraft are run off the air data computers linked to the 1st and second static systems
The vibrating thingy requires power and also so does the encoder electronics bit. And of course your light which to be honest is proberly on a seperate circuit being powered by your flight director alt select logic.
If you go into the back of them a sensitive alt has a din plug fitting and static in the back of them and the unsensitive ones just have static input.
We used have both types in pieces in the flying school and the sentive one/encoder has a whole heap of gubbins/wires that the none sensitive ones don't have. The old ones had a wee out of balance motor and the new ones have a pizo electric vibrator.
And to add the standby alt in me fancy EFIS turboprop isn't a sensitive one and only has one static input from the 3rd static port. All the others on the aircraft are run off the air data computers linked to the 1st and second static systems
Last edited by mad_jock; 14th Oct 2010 at 10:22.
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Counter-drum altimeters
The requirement to have a counter-drum altimeter as opposed to a multi-pointer altimeter is set out in OPS1.652 for commercial air transport. So if you're operating your PA34 for air-taxi, you should have had a counter-drum altimeter since 2002 (and loads haven't!).
There are many non-servoed counter-drum altimeters - Mikrotechna, Aerosonic etc, and the vibrator is essential - without it operating, errors of upto 500ft are not uncommon. Units such as in the picture above (KEA346?) are fully-servoed, and there are also remote versions driven by an air data computer.
There are many non-servoed counter-drum altimeters - Mikrotechna, Aerosonic etc, and the vibrator is essential - without it operating, errors of upto 500ft are not uncommon. Units such as in the picture above (KEA346?) are fully-servoed, and there are also remote versions driven by an air data computer.
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At the risk of adding more confusion, rather than resolution here, I will interject the following thoughts. These thoughts are relative to static driven instruments, not air data, electronic, and repeater types:
An altimeter being "sensative" or not, is a reference to it's resolution of indicaton of altitude. I would be amazed to hear that anyone posting here has flown a certified aircraft with a non sensative altimeter. They've been "gone" for decades, as far as I know. If you have a non sensative altimeter, one or two characteristics will be apparent; the finest resolution is 100 feet, rather than 20 feet, and the indication of 1000 feet of altitude change may be other than one whole turn of the "hundreds" pointer.
As for the encoding altimeters, though I'm sure there are many "new" designs, the encoding altimeters (as opposed to blind encoders) we will likely find in our light aircraft generally have a glass disc which looks like a tiny CD at the back of the instrument. This disc is read similarly to how a CD is read, and that is where the alitiude information comes from. If anything, this disc would damp out errant pointer motion.
I would expect (based upon my knowledge of North American requirements) that aircraft equipped with encoding altimeters, and mode C transponders must have annual or biannual altimeter calibration. Certainly a flaw in the operation of the altimeter itself would become apparent during such recertification. A non sensative altimeter would not be approvable in an aircraft equipped with mode C.
If you have a 500' difference in indicated altitude between two altimeters in one aircraft, at least one of them is not airworthy.
As for the original question, the altimeter in question needs a visit to the instrument shop. Ther are internal opposing forces designed into the instrument (steam gauge types) to cancel out such indications. A part of one of these mechanizms is not working as intended. The altimeter may still read accuractely when not wandering, but requires fixing anyway. There is nothing to be done outside the instrument, as long as you are sure that there is no static blockage.
There was a reference to applying air pressure to the aircraft's static (or pitot) plumbing. NEVER! unless ALL pitot static instruments are completely disconnected. It is not possible to regulate a compressed air source finely enough to not damage an instrument. Recall that half of our 14.7 PSI atmosphere is below 18,500 feet. That means that at 18,500 feet, the pressure is about 7.35 PSI. That's 18 1/2 turns of your hundreds pointer. so about half a PSI is needed to make it go 'round once. Can you regulate the pressure source to 1/2 PSI? Can you even measure it? Use care with instrument handling....
An altimeter being "sensative" or not, is a reference to it's resolution of indicaton of altitude. I would be amazed to hear that anyone posting here has flown a certified aircraft with a non sensative altimeter. They've been "gone" for decades, as far as I know. If you have a non sensative altimeter, one or two characteristics will be apparent; the finest resolution is 100 feet, rather than 20 feet, and the indication of 1000 feet of altitude change may be other than one whole turn of the "hundreds" pointer.
As for the encoding altimeters, though I'm sure there are many "new" designs, the encoding altimeters (as opposed to blind encoders) we will likely find in our light aircraft generally have a glass disc which looks like a tiny CD at the back of the instrument. This disc is read similarly to how a CD is read, and that is where the alitiude information comes from. If anything, this disc would damp out errant pointer motion.
I would expect (based upon my knowledge of North American requirements) that aircraft equipped with encoding altimeters, and mode C transponders must have annual or biannual altimeter calibration. Certainly a flaw in the operation of the altimeter itself would become apparent during such recertification. A non sensative altimeter would not be approvable in an aircraft equipped with mode C.
If you have a 500' difference in indicated altitude between two altimeters in one aircraft, at least one of them is not airworthy.
As for the original question, the altimeter in question needs a visit to the instrument shop. Ther are internal opposing forces designed into the instrument (steam gauge types) to cancel out such indications. A part of one of these mechanizms is not working as intended. The altimeter may still read accuractely when not wandering, but requires fixing anyway. There is nothing to be done outside the instrument, as long as you are sure that there is no static blockage.
There was a reference to applying air pressure to the aircraft's static (or pitot) plumbing. NEVER! unless ALL pitot static instruments are completely disconnected. It is not possible to regulate a compressed air source finely enough to not damage an instrument. Recall that half of our 14.7 PSI atmosphere is below 18,500 feet. That means that at 18,500 feet, the pressure is about 7.35 PSI. That's 18 1/2 turns of your hundreds pointer. so about half a PSI is needed to make it go 'round once. Can you regulate the pressure source to 1/2 PSI? Can you even measure it? Use care with instrument handling....
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It is actually vacuum, not pressure, that is needed to move the altimeter(s).
It is suprising what altitude can be achieved by simply sucking on one of the static vents - 20,000ft no problem. One needs somebody to hold a finger over the other vent, of course.
I doubt this does the instruments much good though, with the altimeter going round 20 times very quickly.
It is suprising what altitude can be achieved by simply sucking on one of the static vents - 20,000ft no problem. One needs somebody to hold a finger over the other vent, of course.
I doubt this does the instruments much good though, with the altimeter going round 20 times very quickly.
