The pressure altimeter
Navigation · Chapter 22 · DGCA CPL and ATPL
How sealed capsules and gears turn pressure into feet, what QNH, QFE and 1013 measure from, and the errors that put you lower than you think.
Remember
- Sealed aneroid capsules (partial vacuum) in a case fed with static; they swell as you climb.
- Pointers: thin + triangle 10 000s, fat 1 000s, long 100s (gears 100 : 10 : 1).
- QFE → height (0 on the ground); QNH → altitude (elevation on the ground); 1013 → FL. Mode C always 1013.
- Reading changes by (new − old setting) × 30 ft; QNH − QFE = elevation ÷ 30.
- High to low (pressure or temperature): over-reads, you are lower. Cold: 4 ft/°C/1000 ft.
How does a box of springs and capsules know your height?
A barometer in feet
- Sealed capsules • Two aneroid capsules, almost no air inside • A leaf spring stops them collapsing
- Static fills the case • The case is fed from the static port only • Fewer specks in the case = thinner air
- Climb: they swell • Outside pressure falls → capsules swell • The push rod lifts the lever, the sector turns the gears
- Gears 100 : 10 : 1 • One turn of the long pointer = 1 000 ft • Two gear pairs slow it down for the other two pointers • The scale is drawn from ISA: 0 ft at 1013.25 hPa (29.92 inHg)
Why are there three kinds of altimeter?
Simple, sensitive, servo
- Simple • Simple altimeter: one capsule, one pointer. Hard to read closely
- Sensitive • Sensitive altimeter: two or three capsules, three pointers, jewelled bearings • A vibrator knocks the friction away, and a subscale lets you set the pressure
- Servo • Servo altimeter: the capsules only move a small I-bar between pick-off coils • A motor drives the pointers: almost no lag below 10 000 ft/min
Three pointers: how do you read the height?
Reading the dial
This lesson is part of the Navigation plan. See plans
The sketch: why does a sealed oval read height?
The S + D box
This lesson is part of the Navigation plan. See plans
Five words, five datums: which is which?
Height, altitude, elevation
This lesson is part of the Navigation plan. See plans
What does each setting make the altimeter read?
QNH, QFE and 1013
This lesson is part of the Navigation plan. See plans
How far apart are QNH and QFE?
QNH and QFE move together
This lesson is part of the Navigation plan. See plans
Solve it: what does it read after you turn the knob?
Solve it: a new setting
This lesson is part of the Navigation plan. See plans
Fly from high to low pressure without re-setting: where do you end up?
High to low, look out below
This lesson is part of the Navigation plan. See plans
Why is cold air dangerous over mountains?
Cold air over-reads
This lesson is part of the Navigation plan. See plans
Solve it: true altitude in cold air
Solve it: cold-air correction
This lesson is part of the Navigation plan. See plans
What else makes an altimeter wrong, and how do you check it?
Other errors and the check
This lesson is part of the Navigation plan. See plans
The static vent ices: what does the altimeter do?
Blocked static
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 22
Test yourself
This lesson is part of the Navigation plan. See plans