Altimeter, VSI, Magnetic Compass

Question 1

what is an altimeter?

Answer 1

a barometer that’s calibrated to read atmospheric pressure as an altitude. (height above sea level)

Question 2

what is the operation of a pressure altimeter?

Answer 2

inside are aneroid wafers that are sealed. air cannot get into, or escape from, them. manufactured to have an internal pressure of 1013.2

static pressure is allowed inside the instrument case and so exerts pressure on the wafers. The higher the pressure the more it compresses, the lower the pressure the more it can expand.

wafers are connected to the hands of altimeter so as it expands or contracts it moves.

Question 3

what do the 3 pointers indicate?

Answer 3

• largest, indicates hundreds of feet ( 1 resolution = 1000ft)
• smaller pointer indicated thousands of feet (1 revolution = 10,000ft)
• skinny with a tip indicated tens of thousands of feet ( 1 resolution = 100,000ft)

Question 4

two-pointer altimeters indicate what?

Answer 4

thousands of feet and large hand for hundreds.

Question 5

what happens to the altimeter in an unpressurised or pressurised cabin with a blockage?

Answer 5

unpressurised - altimeter will overread as the cabin altitude is usually a bit higher than outside.

pressurised - altimeter will match the cabin altitude, which for a light twin aircraft will be around 7000ft lower.

Question 6

what causes instrument error in the altimeter?

Answer 6

small imperfections arising from the manufacture, installation, maintenance and age of individual altimeter instruments.

Question 7

what causes position error in the altimeter?

Answer 7

as the altimeter relies on air from the static vent anything affecting air entering the static source affects the altimeter.

Question 8

what happens to the altimeter with a block or a leakage?

Answer 8

any blockage means the altimeter will not record any change in altitude.

if there’s a leak in the one between the static port and altimeter, it will sense the static pressure at that leak.

Question 9

what causes lag error in the altimeter?

Answer 9

altimeter takes a second or two to respond to rapid pressure changes.

Question 10

what causes barometric error in the altimeter? and how is it overcome?

Answer 10

flying from low to high pressure, altimeter reads low. and the opposite for high to low.

this causes the altimeter to read original altitude and you calculate you have sufficient clearance above this high ground but in fact you’re descending towards it.

overcome by changing the subscale setting knob to surface pressures over your route.

Question 11

what is the serviceability for the altimeter? (4)

Answer 11

• glass checked for damage
• rotation of the adjustment knob produces a corresponding change in altimeter indication
• appropriate MSL static pressure set in the kollsman window
• limits of accuracy of an altimeter read within +30 to - 45ft of the correct altitude when aircraft is on the ground.

Question 12

when local MSL static pressure is set in kollsman window altimeter is said to be set to what?

Answer 12

QNH

Question 13

what is QNH?

Answer 13

the pressure calculated to exist at MSL.

which is obtained by measuring the surface pressure at an airfield datum then converting that to an MSL pressure using specifications of ICAO standard atmosphere.

Question 14

Till what height is QNH used in NZ?

Answer 14

all altitudes below 13,000 ft with the country divided into QNH zones. climbing through 13,000ft 1013hPa should be set to altimeter.

Question 15

what is QFE?

Answer 15

the current pressure level at any datum (mainly aerodromes) other than MSL

Question 16

when is QFE useful?

Answer 16

when the height above airfield is specifically required to be known.

the altimeter is set to be zero feet whilst on the ground and then indicates height above that point, returning to zero upon lading on the same airfield.

Question 17

What is QNE?

Answer 17

the setting of 1013 as the reference pressure.

Question 18

when is QNE useful?

Answer 18

when obtaining pressure altitudes on the ground for planning purposes and in NZ all flights above FL150 which are required to be conducted with QNE and are referred to as Flight levels.

Question 19

what is the construction of a vertical speed indicator?

Answer 19

consists of an aneroid capsule connected to the static system and a sealed case which is also connected to the static system through a calibrated metering unit.

Question 20

what is the serviceability of the VSI? (3)

Answer 20

• check for broken glass
• needle rests at zero - in flight can be checked by timing a climb, if it takes 2 minutes for 1000ft the aircraft is climbing at 500t per minute
• limits of accuracy of VSI are +/- 200ft per minute within a temp range of -20 degrees to 50 degrees

Question 21

what causes lag error in the VSI?

Answer 21

inherent to the operating principle of the instrument.

the indication must be allowed to settle before reading can be considered accurate.

Question 22

what causes position error in the VSI?

Answer 22

a limitation of the design of the instrument. Any action which affects the airflow past the static vent will cause a fluctuation.

Question 23

what happens when static vents are blocked?

and what happens when there’s a leak?

Answer 23

VSI returns to and remains at 0 value.

Leaking in the static system has no appreciable effect on VSI as cabin air pressure changes at about the same rate as ambient.

in pressurised aircraft, VSI will indicate rate of change of cabin pressure.

Question 24

what is the transition layer?

Answer 24

the layer in which aircraft are climbing at 13,000ft changes to QNE and those descending change to QNH at FL150.

Question 25

what if the zone QNH is less than 980hPa?

Answer 25

the minimum usable FL is 160.

Question 26

how does the VSI work?

Answer 26

measures the rate of change of air pressure to gauge whether or not the aircraft is climbing or descending.

then converts the measurements of this change into a number of feet.

Question 27

what is the operation of the VSI?

Answer 27

VSI receives air form only the static port. the inside of the diaphragm is connected to the static port.

the area outside the diaphragm but still inside the instrument case is also connected to the static port but through a metering unit.

Question 28

what does the metering unit do?

Answer 28

measures how fast the aircraft is climbing or descending, that is the rate of change in air pressure.

Question 29

what happens to the VSI if the aircraft is descending ( in which case it is entering air that has a higher static pressure)?

Answer 29

pressure in the instrument case is initially higher than that in the diaphragm due to the metering unit restricting the speed at which air can enter the diaphragm.

causing the diaphragm to decrease in size and this movement is translated into movement of the needle.

Question 30

what happens to the VSI in the climb? (entering air has a lower static pressure)

Answer 30

air leaves the instrument case more easily than the diaphragm causing it to increase in size and again the movement is registered by the dial.

Question 31

once the static pressure is the same in the diaphragm and instrument case what will the VSI read?

Answer 31

zero

Question 32

what causes instrument error in VSI?

Answer 32

within a manufactured item, there’ll be errors. over time through wear and tear more will develop.