Pressure Altimeters – Part 1


For Sophie

Let’s talk altimeters, I am assuming you already understand altimeter settings such as QNH, QNE, QFE & Regional QNH.

The pressure altimeter is a just an aneroid capsule (as found in a weather barometer) inside an air tight box. The capsule is connected to levers which present the pilot with an indication of height or altitude.

Most altimeters are 3 lever instruments although most students never seem to be taught about the 3rd lever which indicates tens of thousands.

Most 3 lever altimeters have a 10,00 feet window which shews hatching when the aircraft is below 10,000 feet.

Although the altimeter box is airtight it has a tapping which takes a pipe (two in some aircraft) to the static vent or port(s) which is normally located on the side of the fuselage although it can sometimes form part of the pitot tube assembly as in the Piper Cherokees.Pitot tubes are dealt with under airspeed indicators.

The static vent also provides static air pressure to the other two pressure instruments, the VSI & ASI. The static vent is positioned at a point on the side of the aircraft which will give static pressure even though the aircraft is in the air flying at speed. To work correctly, the hole (static vent), must be clear and the area immediately around it must be smooth (no paint peeling or surface roughness).

Ice blockage is very serious and a major hazard to a static vent and a pressure altimeter.

Any total blockage will render the altimeter useless as the rest of the system is pressure tight. If a system blockage occurs at 2000 feet the altimeter will continue to read 2000 feet despite whatever altitude the aircraft  climbs or descends to.

This is a three point altimeter shewing 10,180 with the 10,000ft hatching INCORRECTLY shewing. This should not be visible above 10,000 feet.

Some systems are fitted with what is called an alternative static source, in the Piper Aztec for instance it is below and underneath the instrument panel. An alternate static source, if fitted, must be in the cockpit so the pilot can reach it. In the absence of an alternate  static source the glass on the VSI can be levered out with a screwdriver or even broken. This allows static cockpit pressure into the static system containing the altimeter, VSI and ASI. This action would only normally be a last resource when flying in IMC, you would be very unpopular at the flying school if you did this while flying circuits!

Notice on the altimeter on the right that its barometric subscale is in INCHES this is the setting that all American aircraft use. In the UK and Europe we use millibars although after the 17th November 2011 millibars with be replaced by HECTOPASCAL (hPa). As 1 millibar = 1 hectopascal it is only a word replacement.

ALTIMETER PRE-FLIGHT CHECKS

During the preflight inspection any covers over the pitot tube and static vent should be removed and the orifices checked as far as possible for blockage. Remember insects can get into these holes and as I write this there is a mysterious case of sabotage being alleged at an airfield where a pitot tube has been found with glue in it!

Inside the cockpit we check the glasses of all the instruments and their faces for obvious damage.

Then:

Obtain the latest QNH from ATC and check the altimeter against the published altimeter checking point, which hopefully you will be sitting on! This point is normally the apron but information of where it is and how high it is is found in the IAIP AGA 2 – 8 or what in simple days of old was called the UK Air Pilot. You can access this information via the NATS website.

http://www.nats.co.uk/

http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=165&Itemid=3.html

The National Air Traffic website is living proof of how to make a website as user unfriendly as possible. It’s little wonder guides published by Pooleys etc. are so popular as trying to navigate this site takes quite a lot of practice. Still persevere because this is the definitive document and you need to be familiar with it if you take aviation seriously!

You need the aerodrome information-specific page then select your aerodrome = SHOREHAM. Then go to AGA 2.8 and you will see the elevation of the altimeter check point and its position. It is the apron and it is 6 feet amsl. Note, for smaller airfields like  Shobdon & Sleap there is no designated altimeter checkpoint you just have to base it on the aerodrome reference point (usually the highest point on the manoevring area) and that is the elevation shewn on your aeronautical chart.

Your altimeter should be within tolerance. Various organisations have different ideas of what these tolerances should be. The FAA says that a pressure altimeter should be within + or – 75 feet, although I have also seen figures of 25 feet and 85 feet used too. 75 feet is what we use. Your flying school, if it’s a decent one, should have the limits they wish to use published somewhere but my guess is if you ask 3 different instructors you will probably get three different answers!

For a first flight of the day check which is called a  CHECK A we would also check:

SUBSCALE INCREASE HEIGHT INCREASE AND SUBSCALE DECREASE HEIGHT DECREASE

Say it as you do it.

Then set your altimeter to zero and note the reading off the barometric scale, lets say it’s  1000 hPa, then move it to 10 hPa higher to 1010 hPa and your altimeter should now read between 270 and 300 feet.
The reason for that is that in the standard atmosphere at sea level a hPa is = to approximately 27 feet. 30 feet is usually used for calculations in examinations. You do need to understand that relationship, so learn it!

If none of this has been pointed out to you at this stage of your flying instruction in the circuit you can be assured that you school is not doing its job properly.

In part two I will discuss altimeter errors

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One thought on “Pressure Altimeters – Part 1

  1. Captain Jon,
    this is great guide! very well explained firstly about the altimeter itself on whats on it followed by how to make sure to check it properly in order to avoid a disaster. Looking forward for errors bit. Static vent is certainly the most important bit to be checked I guess or else wrong readings for a new pilot with lack of visual understanding may not be a good idea in sky!.. great ..thanks for writing it !

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