Question re altimeters

[planet47]

What is the difference between a sensitive and a non sensitive altimeter?

 

 

[Rastus]

Sensitive usually has 2/3 pointers with the longest indicating 1000ft per revolution.

 

Non-sensitive usually has only 1 pointer and may indicate as much as 10000ft per revolution.

 

Then there are the ones that have 2 pointers and the long one indicates 3000ft per revolution. Found mostly in Eastern European gliders.

 

Robert

 

 

[facthunter]

A sensitive altimeter has a small vibrator in it powered by electricity. This is to overcome the initial stiction of the mechanism. The small movement of the baro diaphragm is magnified massively to produce a reading. Often you will. have gone several hundred feet before you get an indication. It's more important in Instrument flying to have a quick response and also when gliding when you are trying to detect thermals. Nev

 

 

[Dafydd Llewellyn]

A sensitive altimeter has a small vibrator in it powered by electricity. This is to overcome the initial stiction of the mechanism. The small movement of the baro diaphragm is magnified massively to produce a reading. Often you will. have gone several hundred feet before you get an indication. It's more important in Instrument flying to have a quick response and also when gliding when you are trying to detect thermals. Nev

That's news to me; what brand has that? Must be a glider instrument; normal sensitive altimeters get sufficient vibration from the engine. Gliders normally uses a variometer (fast-response rate-of-climb), with total-energy compensation, to detect thermals.

 

 

[facthunter]

All the Jet ones have them, as they don't vibrate much. You check them by putting the microphone (on intercom) on them and clicking the button. You can hear the rattling noise in your headset. A piston engine single wouldn't need that sophistication, for the reason you state. Nev

 

 

[planet47]

Which one would I be more likely to use in an ultralight?

 

 

[Dafydd Llewellyn]

Sensitive usually has 2/3 pointers with the longest indicating 1000ft per revolution.Non-sensitive usually has only 1 pointer and may indicate as much as 10000ft per revolution.

 

Then there are the ones that have 2 pointers and the long one indicates 3000ft per revolution. Found mostly in Eastern European gliders.

 

Robert

Yep. The 3000 ft / revolution ones are an adaption of 1000 metre/revolution.

 

All the Jet ones have them, as they don't vibrate much. You check them by putting the microphone (on intercom) on them and clicking the button. You can hear the rattling noise in your headset. A piston engine single wouldn't need that sophistication, for the reason you state. Nev

Ta - I don't get to play with that kind of hardware. In a glider, one normally taps the altimeter with a finger to see which way the needle moves, if you're in marginal lift. But I don't think that the incorporation of a vibrator defines a "sensitive" altimeter; if it reads 1000 feet per turn, it's a "sensitive" altimeter.

 

 

[Dafydd Llewellyn]

Which one would I be more likely to use in an ultralight?

I don't think the non-sensitive ones are acceptable. See CAO 20.18

 

 

[facthunter]

If you use the more needles description., which may be the commonly accepted term, and I'm not convinced it's universal. In the past I think the simpler dial altimeter was acceptable in RAA. The altimeter is one of the worst designed instruments fitted to an aeroplane. Misreading and miss setting them has caused (and will continue to cause) major accidents. More so in IF conditions, where it is more relied on.

 

 

[frank marriott]

The only single needle altimeter I have seen up close didn't have provision for an external static port.

 

 

[facthunter]

It's unlikely any of that stuff is TSO'd. If you are going at much speed at all and the cockpit is enclosed you need a static port. It should be remembered that the common altimeter does not read true height above sea level. The actual altitude registered varies depending on the atmospheric conditions. The whole calibration is based on a "standard atmosphere" assumption, and separation depends on the instrument being within tolerences and the Kollsman scale being set appropriately and everyone else doing the same at the time you are flying. Nev

 

 

[Guest Andys@coffs]

My suggestion is that its true that much of the Ultralight fraternity Altimeters are not TSO'd.....My experience with "Falcon" brand Altimeters (Chinese made) is that you might well be significantly better off buying a 2nd hand TSO'd altimeter than a Falcon brand as an example......By the time you've replaced your 2nd Falcon altimeter, and knowing that the 3rd is just a few hours down track.....you may well be better having the TSO'd instrument knowing that if it fails parts, manuals and methods of repair exist and can be called on. whereas with the falcon all that's available is the sound of crickets from some undeterminable altitude.......

 

In my aircraft I use an MGL electronic altimeter that relys on the distortion of a quartz crystal (piezo affect) as pressure between reference and static changes. As Quartz crystal distorts a voltage is produced that is proportionate to the amount of distortion. That menas that the concept of mechanical inertia that needs to be overcome...usually by tapping becomes moot.

 

Andy

 

 

[Dafydd Llewellyn]

Civil Aviation Order 20.18

 

Appendix I

 

Instruments required for flight under Visual Flight Rules

 

(Limited to aircraft specified in subsection 3 paragraph 3.1)

 

1 The flight and navigational instruments required for flights under the Visual Flight

 

Rules are:

 

(a) an airspeed indicating system; and

 

(b) an altimeter, with a readily adjustable pressure datum setting scale graduated in

 

millibars; and

 

© (i) a direct reading magnetic compass; or

 

(ii) a remote indicating compass and a standby direct reading magnetic

 

compass; and

 

(d) an accurate timepiece indicating the time in hours, minutes and seconds. This

 

may be carried on the person of the pilot or navigator.

 

2 In addition to the instruments required under paragraph 1, aircraft, other than

 

helicopters, engaged in charter or aerial work operations and operating under the

 

Visual Flight Rules, shall be equipped with:

 

(a) a turn and slip indicator (agricultural aeroplanes may be equipped with a slip

 

indicator only); and

 

(b) an outside air temperature indicator when operating from an aerodrome at which

 

ambient air temperature is not available from ground-based instruments.

 

Appendix IV

 

Instruments required for aeroplanes engaged in:

 

(i) aerial work and private operations under the Instrument Flight Rules

 

(including night V.M.C.); and

 

(ii) charter operations under night V.M.C; and

 

(iii) Instrument Flight Rules freight only charter operations in aeroplanes

 

with maximum take-off weight not greater than 5 700 kg.

 

1 The flight and navigational instruments required are:

 

(a) an airspeed indicating system; and

 

(b) a sensitive pressure altimeter; and

 

© (i) direct reading magnetic compass; or

 

(ii) a remote indicating compass and a standby direct reading magnetic

 

compass; and

 

(d) an accurate timepiece indicating the time in hours, minutes and seconds, except

 

that this may be omitted if it is carried on the person of the pilot or navigator; and

 

(e) a rate of climb and descent indicator (vertical speed indicator) for other than night

 

V.M.C. flights; and

 

(f) an outside air temperature indicator; and

 

(g) an attitude indicator (artificial horizon); and

 

(h) a heading indicator (directional gyroscope); and

 

(i) a turn and slip indicator except that only a slip indicator is required when a

 

second attitude indicator usable through flight attitudes of 360 degrees of pitch

 

and roll is installed; and

 

(j) means of indicating whether the power supply to the gyroscopic instruments is

 

working satisfactorily; and

 

(k) except for aeroplanes engaged in night V.M.C. flights, means of preventing

 

malfunctioning due to either condensation or icing of at least 1 airspeed

 

indicating system.

 

2 The instruments specified in subparagraphs 1 (a), (b), (e) and (k) of this Appendix

 

shall be capable of being connected to either a normal or an alternate static source but

 

not both sources simultaneously. Alternatively, they may be connected to a balanced

 

pair of flush static ports.

 

3 Except for aeroplanes engaged in night V.M.C. private and aerial work operations the

 

instruments specified in subparagraphs 1 (g), (h) and (i) of this Appendix shall have

 

duplicated sources of power supply unless the turn and slip indicator or the second

 

attitude indicator specified in subparagraph 1 (i) has a source of power independent of

 

the power operating other gyroscopic instruments.

 

4 A gyro-magnetic type of remote indicating compass installed to meet the requirements

 

of subparagraph 1 © (ii) of this Appendix may be considered also to meet the

 

requirement for a heading indicator specified in subparagraph 1 (h) of this Appendix,

 

provided that such installation complies with the power supply requirements of

 

paragraph 3 of this Appendix.

 

 

[dutchroll]

The distinction between a "sensitive" and "non sensitive" altimeter is often muddled.

 

Big commercial jets have sensitive altimeters (required for RPT operations), yet often use a drum altimeter with only one pointer, or an electronic display with a moving altitude tape and fixed indent.

 

The critical difference is the provision in the "sensitive" altimeter for adjustment of the barometric datum - or in other words, the Kollsman Window or its glass cockpit equivalent.

 

Once upon a time, recognising a sensitive altimeter by its use of more than one pointer was an easy distinction, as these by default had the Kollsman Window and old style single pointers did not. However these days the number of pointers isn't relevant to whether it's "sensitive" or "non sensitive" (nor is the altitude scale or resolution, which some people also mistake as a defining characteristic of a sensitive altimeter).

 

 

[jakej]

..and don't buy the single pointer types of Altimeters (not approved) as any Instrument test guy worth their money won't give it the tick of approval. Same sort of thing goes with the Falcon or Wultrad units, can't be legally repaired when they fail, which is only a matter of time - very short time as I've seen them failed straight out of the box.

 

Jake J