Altimeters

[Pilot Pete]

Is there any reason we can't use digital Altimeters in our homebuilts? I can pick these up fairly cheap.

 

 

[Pilot Pete]

The type I have looked at is a combination altimeter,compass and temp and is about the size of a mobile phone.

 

 

[facthunter]

Altimeters. Suitability.

 

IF the data is presented as a series of numbers, it would have shortcomings as to readability, especially if you are climbing/descending rapidly.

 

If you were trying to hold a set altitude sat 4,000 feet to a tolerance of 100' I would suggest that interpreting a series of changing numbers would be far more difficult than watching a needle increasing or decreasing over the 100's feet acale.

 

You also need to be able to set QNH, and standard pressure 1013.2 on the Kollsman scale, when appropriate.

 

There is a serviceability test (Max. error permitted) for a tso'd (approved) unit.

 

If the unit you have is GNSS (GPS) supported it is not a Barometric instrument calibrated to the required standard atmosphere so would not read a level based on pressure so you would not have an instrument compatible with what all other users would have. Same problem with using an altitude based on you GPS. It might be quite accurate, but it is not what all the other traffic are using. They fly a pressure height, not a physical height.

 

Nev

 

 

[Pilot Pete]

So what you are saying is that I could be flying at 1000ft according to the digital altimeter and be flying at a different height according to a barametric altimeter, thereby putting two aircraft at risk because each measuring device is giving a different reading. Much safer to fly with an analouge instument too I guess,one that does not require a power source to function.

 

So if a Normal instrument is the accepted way to go , what is the difference between a plain altimeter and an Encoding Altimeter?

 

 

[Pilot Pete]

Dont worry about the answer, I found out. An Encoding altimeter is used to send info to a transponder so the likes of a control tower can read your height

 

 

[eightyknots]

As facthunter said, there is a real readability problem with digital readouts.

 

The issue with that is, during a rapid change (in this case ascent or descent), the unit "samples" and then displays the output in digital form and then samples for the next display a second or so later. This means that anything displayed is really historical, i.e., it has already happened. This could become a problem (a) during quick descents, and (b) when trying to keep a steady height. By the time you see it, it is no longer the actual height.

 

The second matter is that we are able to interpret needle positions more quickly than rapidly changing numbers on a screen. During the 1970s, digital watches became all the rage. Now, most watches sold are analogue for that reason: it's easier on our brain to see that it is "ten past" by the position of the hands than 12.10 on a digital readout.

 

 

[eastmeg2]

So long as it's a reasonably accurate barometric altimeter with adjustable QNH setting it's hard to see why it couldn't be used in CAO-95.XX aircraft, though I expect Steve Bell would have the final word.

 

It's my understanding that CAO-95.XX aircraft are not required to use TSO'd altimeters and that the Chinese made altimeters that exist in many of our recreational aircraft (factory and ametuer built) are not TSO'd.

 

As for GPS altitude readouts, don't put any faith in any instantaneous readings. I've seen my Garmin 296 (version 5.7 firmware) giving sporadic altitudes with wildly changing errors exceeding 500ft above and below my aircraft altimeter reading - in smooth air I might add. I have had instances of being 500ft below CLL and my Garmin 296 briefly telling me I'm VCA.

 

On most occasions the Garmin 296 is fairly stable and accurate but sometimes it's not. Just yesterday when I saw it changing wildly I checked the satellite reception page and I had between 4 to 7 strong satellite signals. Maybe the satellites were having a bad day.

 

Cheers,

 

Glen

 

 

[The Wolf]

The issue with that is, during a rapid change (in this case ascent or descent), the unit "samples" and then displays the output in digital form and then samples for the next display a second or so later. This means that anything displayed is really historical, i.e., it has already happened. This could become a problem (a) during quick descents, and (b) when trying to keep a steady height. By the time you see it, it is no longer the actual height.

Cant say i agree fully with that. There is still a mechanical lag when using analouge gauges, so if you are rapidly decending/accending you are still seeing the historical height. The analogue gauge would actually be slower to respond than the digital gauge, because of all the friction and force required to move the needle. Because the reading isnt flashing and requiring your brain to compute the new numbers, it is only giving you the perception that it is working better.

 

Digital would still be harder to read and interpret though.

 

 

[facthunter]

Lag in instruments

 

This is called "hysteresis" (I think). There is stiction in the movement . A good example is with a VSI. (vertical speed indicator). These are a handy instrument when flying without visual reference to the ground etc. However if they are sluggish to change indication then your reaction which may be a little sluggish also adds to it and you don't manage holding your height very well.

 

SO they invented a more expensive VSI called an IVSI. To reduce the lag in reading there is a vibrator inside the instrument which shakes the mechanism and makes the indication more responsive. Instantaneous is what "I' stands for. (Slight exaggeration, but a great improvement). Nev

 

 

[Guest davidh10]

Cheap Chinese Altimeter give "instantaneous" reading by tapping a forefinger on the glass

Gee. Been doing that since time immemorial with any make of aneroid barometer..

 

That reminds me. Physics question: How do you measure the height of a building with a barometer? After failing the question, the teacher asked the student, who was pretty bright, why he had given a wrong answer. The student's reply: Actually there are three methods:-

 

1. Take the barometer to the top of the building and drop it off the side. Time how long it takes to get to the bottom and knowing the value of gravitational force you can calculate the height.
    
    
   
2. Go the the Janitor (obviously an American student) and say "I have a lovely barometer here and it is yours if you tell me the height of the building.
    
    
   
3. Measure the barometric pressure at the bottom and top of the building and calculate the height from the difference.
    
    
   

 

 

 

 

Pete;

 

The device you have shown is for orienteering / mountain climbing etc. not for aviation. This may also mean that it samples less often as a person is not likely to change altitude in these pursuits as quickly as in an aircraft. It does use a pressure transducer to measure barometric pressure however as said by someone else, you need to correct the altitude readout by changing the pressure reading (QNH). You may need to adjust for correct altitude of the runway while on the ground, or to set a specified QNH while airbourne. The compass uses a magnetometer, so it will be influenced by iron or magnets in proximity. This means that the compass may not be accurate depending on where it is located in the aircraft. That may require adjustment, but it also means mounting it and not having it around your neck or moving about.

 

While the instrument may have the requisite controls, accuracy and displays, it is also a question of how easy it is to use in flight. You don't want to be messing about with a lot of buttons and menus just to get basic instrument information. The price may be an indication of quality too at US$14 !!

 

The fact that it is battery operated (2 x AAA batteries) is also a disadvantage, as a flat battery will deprive you of a basic flight instrument. Back lighting is also very handy if you fly close to last light, and you don't want to have to push a button for momentary lighting. Better to run off the aircraft power system.

 

As to digital or analogue readouts.. My aircraft has both (all electronic on an LCD screen) and I tend to use both. The analogue gives a quick indication of trend or status, while the digital gives a much finer grained value that is good for maintaining a constant value or picking up a small change (for example in CHT or EGT).

 

 

[facthunter]

Discuss 1,2& 3.

 

referring to thee above and I know there is some aspect of tongue in cheek, but it may help understanding of the subject.

 

#1. Timing the descent and using the acceleration figure for "G". No good, as you do not have any figures for drag. That method only works in a vacuum.

 

#2. Should work. ( He might be a student working illegally so there is an element of risk ).

 

#3 What values do you use here? The altimeter is based on standard atmosphere. which really never exists. You would need to have the actual values for the column of atmosphere you are dealing with. You can't just say 30' or such per Mb. Nev

 

 

[facthunter]

Still doesn't work.

 

Talking to principles here. You might have those figures provided on the atis or whatever, but that does not guarantee that the pressure/ temp/ density varies as the profile of the ISA. ( International Standard Atmosphere). ( A structured set of values used to calibrate ALL approved altimeters)

 

Barometric height is never an actual height unless by cooincidence. One day we will use GNSS data so that when we fly over mountains we will know the actual height of the plane and (on an appropriate map) the mountain, and therefore what clearance we have from it. Currently the way satellite data is processed, it may not always give accurate Height info, but there is no reason why it can't ( In principle). Nev

 

 

[eightyknots]

Gee. Been doing that since time immemorial with any make of aneroid barometer..That reminds me. Physics question: How do you measure the height of a building with a barometer? After failing the question, the teacher asked the student, who was pretty bright, why he had given a wrong answer. The student's reply: Actually there are three methods:-

 

1. Take the barometer to the top of the building and drop it off the side. Time how long it takes to get to the bottom and knowing the value of gravitational force you can calculate the height.
    
    
   
2. Go the the Janitor (obviously an American student) and say "I have a lovely barometer here and it is yours if you tell me the height of the building.
    
    
   
3. Measure the barometric pressure at the bottom and top of the building and calculate the height from the difference.
    
    
   

How about these for a collection of alternative answers?

 

*Tying the barometer to the end of a piece of string, lowering the barometer from the top of the building and then measuring the length of the string and barometer once the barometer touches the ground.

 

*Lowering the barometer from the top of the building on a piece of string, then swinging it like a pendulum and measuring the period of its oscillation;

 

*Counting the number of stairs climbed whilst carrying the barometer up to the roof and multiplying that by the height of one stair riser.

 

*Placing the barometer against the building at ground level, marking the top, placing the barometer above the mark, marking the new top, and so on until the building has been measured in "barometer units";

 

*Measuring the barometer, finding the length of the shadow cast by the barometer when stood on the ground, then finding the length of the building's shadow in the same conditions.

 

...and then, a less dishonest method:

 

*Chuck it at the teacher, then whilst he is knocked out, read the answer sheet.

 

 

[Pilot Pete]

...and then, a less dishonest method:

 

*Chuck it at the teacher, then whilst he is knocked out, read the answer sheet.

 

This I think is the correct answer. When you take into account the Human Factors elements such as fatigue management and the ability to make judgement calls based on what is the least stressfull and safest way to coduct the experiment,this is by far the better way to go.

 

 

[Guest davidh10]

Thanks Nev and EightyKnots for giving that joke a work out. Seeing Nev's first post, I was about to add the one about the string, but EightyKnots beat me to it :)

 

Nev. You are quite correct in your assertions, but it makes a good story :big_grin: Always a good brain exercise to consider the realities.

 

Pete. I hope you have obtained some useful info on which to base a decision, and some amusement into the bargain :)

 

 

[Pilot Pete]

Yes David, I have learnt a lot. The main thing is that before each flight I must recalibrate the altitude stringy thingy device.

 

All jokes aside , I have been brought over to the Light Side, and the force is now strong within me. I feel I can now make an informed decision on what type of instrument I will be using.:confused:

 

 

[Guest ozzie]

Another thing that you may not have picked up and seems to go unnoticed to newbies and airport operators these days is if you go to a airport and you notice a sign that says the name of the airport and elevation above sea level you can dial up the said elevation on the alimiter and in the subscale you will have the correct QNH. This has gone by the wayside since the introduction of ATIS and ATC.

 

 

[Pilot Pete]

I have thought about that very thing often. Tis why I am trying to be informed as much as possible. I'm not saying that I know everything and I really appreciate the replys and the knowledge given freely. I am definately going the tried and true method of using an analogue altimeter and doing the tap tap tap thing to keep the needles honest.

 

 

[Guest ozzie]

Do not tap any sensitive instrument. they get enough jarring as it is. if you do and it jumps around several hundred feet, like my skydive alti does, you should have it serviced. i'll get around to it soon, or buy a new one soon.