The Altimeter Flashcards
What are the units used for an altimeter?
- Feet
- metres (1m=3.28ft)
Height
Is the vertical distance of the aircraft from a point on the ground. In order to indicate this, the altimeter sub-scale is set to QFE
Altitude
Is the vertical distance of the aircraft above mean sea level (MSL). To indicate altitude, the altimeter sub-scale is set to QNH
Indicated Altitude
Is the altitude shown on the altimeter. Is inaccurate owing to:
-large errors -> temperature error and pressure error
-small errors -> instrument error, position error and static system error
True Altitude
Indicated altitude corrected for barometric error and temperature error
Pressure Altitude
The altitude shown on an altimeter when 1013 its set on the sub-scale. Pressure altitudes are described ass Flight Levels (FL). The pressure altitude minus the last two digits is FL. 10,000ft pressure altitude is FL100
Density Altitude
Is the ISA altitude equivalent to actual pressure altitude in ambient conditions:
DA=PA+(ISA dev x 120)
QFE
Is the pressure measured at ground level at an airfield,. When set to QFE, the altimeter reads approximately zero on touchdown
QNH
The pressure measured at ground level at an airfield, adjusted to sea level using ISA conditions. When set to QNH, the altimeter reads airfield elevation on touchdown
SPS
Standard Pressure Setting (1013hPa). When set to SPS the altimeter shows the pressure altitude
Operating principle of an altimeter
1) a partially evacuated aneroid capsule is contained within a sealed casing. The casing is connected to the static pressure port.
2) Usually a stack of capsules is used to increase the range of movement
3) a spring limits the collapse of the capsules under casing (static) pressure
4) as the aircraft climbs, the case pressure reduces; the capsules expand
5) a suitable system of levers and gears moves the instrument pointer to indicate altitude
The simple altimeter
- one capsule
- one pointer hand
- limited accuracy and range
- no sub scale, so can only be set to zero (QFE) before takeoff
- not good enough for training or commercial aircraft
Sensitive Altimeter
- Stack of aneroid capsules to increase accuracy and operating range
- Multiple pointer hands allow more accurate indication
- potential to be mis-read because of multiple pointer hands
- When equipped with altitude reporting it is the minimum required in controlled airspace
- jewelled bearings and vibration or knocking system reduce friction and sticking to improve accuracy
- remaining friction in the linkage system results in accuracy not being good enough for some categories of airspace
Servo Altimeter
- Stack of aneroid capsules moving on the āIā bar part of an E/I bar transducer.
- almost zero friction means almost no lag
E bar feeds AC to a servo-motor which moves it to realign with the I beam, and at the same time drives the pointer hands and numerical display - this is the minimum standard of altimeter for RVSM airspace
- Always has an altitude reporting facility
Displays for altimeters
Can be:
- Pointer (Simple altimeter)
- Multi-pointer (Sensitive/servo-altimeter)
- Pointer and drum numerical scale (servo altimeter)
- Vertical straight scale (EFIS altitude tape)
Barometric error
Is caused by the altimeter reference pressure not being the same as the local sea level pressure. Corrected using the formula:
True Alt.=Indicated Alt. +(actual hPa - set hPa) x 30
High to low, beware below
Temperature error
Temperature error= 4 x (Indicated Alt./1000) x ISA deviation
Lag error
Friction in the mechanism connecting the aneroid capsule to the pointer hands means that the indicated altitude lags behind actual altitude in climb or descent.
- Reduced by using jewelled bearings for the mechanism
- Reduced by using a vibration/knocking device to prevent static friction
- Removed by E/I bar and servo-motor system (in a servo altimeter)
- Removed by use of MEMs sensors feeding air data computers
Position Error
Is caused by airflow disturbances around the static port.
- Mostly a problem on small aircraft where there are no large areas of fuselage with stable airflow for siting the static ports.
- A correction table may be required to adjust for gear and flap changes
Static System Blockage
Obstructions (plugs, tapes, insects or nests) blocking the static system causes altimeter errors:
- Complete blockage causes altimeter to lock-up
- Partial blockage causes very large altimeter lag:
- under-read in a climb
- over-reads in a descent (particularly dangerous)
Static system leak in pressurised aircraft
Air leaks from cabin into the static system, increasing pressure in the altimeter casing, causing the altimeter to under-read, possibly by a significant amount
Static system leak in unpressurised aircraft
Air leaks from the static system into the cabin, which is slightly below ambient pressure because of the Venturi effect, causing the altimeter to over-read slightly
GPS altitude
- Useful for checking altimeters which are disagreeing, to determine which is correct and which is faulty.
- Not accurate enough for general use so not displayed in an flight instrument
- can be used as a back-up system in some aircraft, in which case labels and alerts remind crew that it is inaccurate
- Integrity and accuracy reduced by limits of satellite coverage, geometrical priority of lateral navigation, and inaccuracies in geodetic modelling of the Earth
