Altimeter is an Aneroid barometer.
i.e. Dry barometer (no liquid).
Altimeter has a partially evacuated capsule which expands/contracts with the movement of static pressure in the atmosphere, this movement of static pressure is picked up by a leaf spring and passed on to on indicator. The assembly of the instrument is kept in an air light case and is fed with Static pressure.
On Climbing the Altimeter setting is changed from QNH to QNE- at Transition Attitude.
On Descending the Altimeter setting is changed from QNE to QNH at Transition Level.
Transition layer layer of air thickness, that varies between 1000 to 1500 feet layers of air between transition altimeter and transition level.
Note:
- If the QNH falls below 997 hp at an Aerodrome the thickness of transition layer is increased to 2000 feet to maintain vertical separation.
- If the QNH is between 1013.2 and 997 hp the transition layer is 1500 feet.
- If the QNH is above 1013.2 the transition layer is reduced to 1000 feet.
Aerodrome Control is the controlling authority below Transition Altitude.
Approach Control is the controlling authority at or above Transition Level.
- Rate of fall of press with height over Cold areas is more as compared to warm areas.
- If temp is less than Standard Altimeter Over reads.
- If temp is more that Standard Altimeter Under reads.
- Flight levels are found at a lower level on cold day.
- In Winter Flight Levels are found at a lower level.
- Therefore, in Winter Aircraft Altimeter Over reads.
- If Static is blocked we can use the Alternate Static source.
- If no Alternate Static source is available, it is advised to break the VSI glass because in such a condition it is the least important equipment.
- In a pressurized a/c if V.S.I. is broken the Cabin press will leak into it and Altimeter will show a descent and indicate the maintained Cabin Altitude.
- Altimeter and power will indicate air speed if V.S.I. is not working.
Encoding Altimeter: Pneumatic type Altimeter is a conventionally barometric altimeter with an encoding device that converts altitude into digitally coded electoral signals in 100 foot increments.
It uses evacuated capsules called aneroid to sense altitude, by expanding and contracting with static pressure.
The movement of the aneroid walls drives the altimeter pointers for visual display via gearing.
At the same time the pressure of the moving aneroid drives a small glass disc providing the elevation signal to activate the encoding device.
Encoding Altimeter
Servo type altimeter uses the aneroid system to sense altitude but the power to drive the encoding and display mechanism is derived from the airplane’s electrical system.
The servo type altimeter having an outside source of power drives pointers and encoders at the same time provide ancillary output for an altitude alerter, rate of climb indicator, vertical navigation system and Autopilot.
The servo altimeter is very accurate and very reliable. It’s disadvantage is that it is completely dependent on electric power (A/c electrical system) to operate.
Hysterisys: is an error of Altimeter, which occurs due to the elastic properties of the capsule. It occurs since the capsule shows a lag in the change of altitude reading. It is very small and can be ignored.
QFE: Aerodrome level pressure on ground altimeter reads 0 feet. In air altimeter reads height above aerodrome level.
QNH:Aerodrome level press reduced to mean sea level in I.S.A. on ground or in air altimeter reads altitude above M.S.L.
Note:
- The different between QNH-QFE of a particular place aerodrome will always be constant.
- Irrespective of pressure and this depends upon elevation of the aerodrome.
- On ground QNH reads airfield elevation above M.S.L.
QNE: Standard setting of 1013.2 or 29.92 of h.p.
On setting QNE Altimeter reads pressure attitude.
Altimeter: On ground capsule contracts.
In a climb to an altitude the capsule expands.
Over cold areas the altimeter over reads
H – L – H Indicated Altitude= True Altitude+ Error
L – H – L Indicated Altitude =True Altitude – Error
When aircraft is flying over cold areas or in winter, altimeter over reads, but actually the aircraft is low. In this case the speed also decreases, since at a low altitude the TAS decreases.
When aircraft is flying over warm areas or in summer, altimeter under reads, but actually the aircraft is high. In this case the speed also increases, since at a high altitude the TAS increases
COLD WEATHER
Cold weather causes high barometric pressure.
Altimeter has maximum scale settings of 31 inches.
Low High Low Altimeter Under reads Indicated Altitude = True Altitude + error
A/c is higher than shown altitude.
From 3I 31.28 A/c altimeter under reads
If you change to 31.28 altimeter readings decrease
High Low High Altimeter over reads Indicated Altitude = True Altitude – error
Flying From 31.28 hp – 31.00 hp A/c altimeter will show over read if you do not change to 31.00 hp altimeter setting, therefore the A/c is lower than the shown altitude.
Therefore If altimeter setting exceeds 31.00 hp all pilots conducting en route operations below 18,000ft maintain 31.00 hp on altimeter setting.
If the altimeter setting increases above 31.00 hp for the destination airport, we increase Ceiling – 100 feet, Visibility by 1/4 statute mile and for each .1 hp above 31.00
Example- for the value of 31.20 hp we add
2 x 100 = 200 feet for the ceiling and. 2 x ¼ = 2/4 statute mile is added to the minimum required visibility.
Altimeter
Static is fed into the case having altimeter capsules.
As A/c climbs value of static decreases and capsule expands.
As A/c descents value of static increases and capsule contracts.
Therefore at sea level when A/c is parked on the ground, capsule is contracted.
Static Block – if Static blocks, the old static will remain blocked (trapped in the case) and height changes will not be indicated.
It static is blocked in flight V.S.I glass may be broken (least important instrument) and cockpit pressure can be fed to the Altimeter.
Alternate pressure source (like cabin pressure) when used in altimeter (the pressure in the most of un pressurized A/c cockpit is less than atmosphere pressure) the altimeter will over read.
QFE –QFE is the barometric pressure at aerodrome level.
QFF – QFF is the barometric pressure at aerodrome level reduced to mean sea level, using actual values of temperature and pressure prevailing at the time and not the ISA values.
This is used by Meteorological offices for plotting synoptic charts.
QNH – QNH is barometric pressure at aerodrome level reduced to mean sea level using ISA formula.
When set on the altimeter, it shows altitude of aerodrome above Mean sea level on ground and altitude of A/c above Mean sea level in the flight.
Conventional Altimeter has one capsule only.
QNH setting is also used for checking altimeter serviceability, Its reading is called attitude.
Two types of QNH –
- Spot QNH – Spot QNH is valid for spot only and is used for Take off and landing as an alternate to QFE.
- Regional QNH – Regional QNH is valid for a region and is used during cross country to maintain adequate terrain clearance.
QNE – Standard alt setting of 1013.2 hp, All A/c flying over an aerodrome above Transition Altitude shall report altitude in level and set 1013.2 hp, on altimeter scale.
All a/c flying below Transition Level shall set their altimeter setting to QNH and report altitude as attitude above Mean Sea Level.
Temperature affects altitude of A/c (True Altitude)
Going from cold to warm – altimeter under reads (True altitude is high)
Going from warm to cold – altimeter over reads (True altitude is low)
Density Altitude is Pressure Altitude, corrected for temperature deviations from standard I.S.A. conditions.
If temperature is higher then I.S.A. conditions, Density Altitude is higher.
If temperature is lower then I.S.A. conditions, Density Altitude is lower.
10 Celsius of temperature difference causes Density Altitude to differ from Pressure Altitude by approximately 120 feet.
SERVO ALTIMETER
It uses two capsules
Mechanical linkage is replaced by Servo Assisted transmission system.
Advantages
- More reliable at high altitudes.
- Power transmission gives accuracy.
- No time lag error.
- Eliminates the possibility of misreading by digital system.
- Very useful at low altitude.