The Altimeter

Question 1

What are the units used for an altimeter?

Answer 1

• Feet
• metres (1m=3.28ft)

Question 2

Height

Answer 2

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

Question 3

Altitude

Answer 3

Is the vertical distance of the aircraft above mean sea level (MSL). To indicate altitude, the altimeter sub-scale is set to QNH

Question 4

Indicated Altitude

Answer 4

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

Question 5

True Altitude

Answer 5

Indicated altitude corrected for barometric error and temperature error

Question 6

Pressure Altitude

Answer 6

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

Question 7

Density Altitude

Answer 7

Is the ISA altitude equivalent to actual pressure altitude in ambient conditions:

DA=PA+(ISA dev x 120)

Question 8

QFE

Answer 8

Is the pressure measured at ground level at an airfield,. When set to QFE, the altimeter reads approximately zero on touchdown

Question 9

QNH

Answer 9

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

Question 10

SPS

Answer 10

Standard Pressure Setting (1013hPa). When set to SPS the altimeter shows the pressure altitude

Question 11

Operating principle of an altimeter

Answer 11

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

Question 12

The simple altimeter

Answer 12

• 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

Question 13

Sensitive Altimeter

Answer 13

• 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

Question 14

Servo Altimeter

Answer 14

• Stack of aneroid capsules moving on the ‘I’ 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

Question 15

Displays for altimeters

Answer 15

Can be:
- Pointer (Simple altimeter)
- Multi-pointer (Sensitive/servo-altimeter)
- Pointer and drum numerical scale (servo altimeter)
- Vertical straight scale (EFIS altitude tape)

Question 16

Barometric error

Answer 16

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

Question 17

Temperature error

Answer 17

Temperature error= 4 x (Indicated Alt./1000) x ISA deviation

Question 18

Lag error

Answer 18

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

Question 19

Position Error

Answer 19

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

Question 20

Static System Blockage

Answer 20

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)

Question 21

Static system leak in pressurised aircraft

Answer 21

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

Question 22

Static system leak in unpressurised aircraft

Answer 22

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

Question 23

GPS altitude

Answer 23

• 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