Section 03 - Altimetry (cont)

Lesson 09

Height Calculations Minimum Safe Altitude Flight Level Transition Altitude Transition Level Transition Layer Further Altimeter Calculations

Altimeter Barometric Error Altimeter error due to changing surface barometric pressure

Terrain Clearance

MINIMUM FLIGHT LEVEL To determine the

minimum safe flight level that we can fly along a particular route, we should need to know the elevation of the highest ground or obstacle along our track, the minimum terrain clearance (varies with company regulations) and the QNH.

Barometric height equivalents

When it is time to adjust altimeter readings into pressure differences or vice versa it is important to know how many feet (or m) correspond to one hPa.

At sea level 1 hPa is equivalent to 27ft or 8m. At 2000 ft above mean sea level (AMSL) 1 hPa is

equivalent to 30 ft. 20000 ft (AMSL) gives 1 hPa= 50 ft and 40000 ft (AMSL) gives 1 hPa = 100 ft.

When doing altimeter calculations use 1mb=27ft.

Vertical Position References

Transition Altitude

Transition Altitude: Is that altitude at or below which we refer to

our vertical position as ‘altitude’ based on QNH. Outside controlled airspace this is usually 3000 ft. Inside controlled airspace it can vary and prior to departure this should always be checked for departure and destination airfields.

Transition Level

Transition Level Is the lowest usable flight level and TL is

determined locally with consideration taken to surrounding terrain, temperature and air pressure.

It is the first 500 ft flight level that is reached after climbing through the transition altitude.

It is also the level on descent when the change is made from 1013mb to QNH.

Transition Layer Is that layer between the Transition Altitude and the Transition Level. Ascending aircraft will be on SPS setting passing the layer. Descending aircraft will be on QNH passing the layer.

Pressure Datums

QNH Less than SPS

QNH Greater than SPS

Terrain and Wind Influence on Pressure Levels

constant2

Equation sBernoulli'2

gzpv

V1

V2

Velocity over terrain increases. Pressure drops (Bernoulli’s Theorem). Height of constant pressure levels over terrain

decreases. Suggested safety margins

Wind <30kt; no addition 31-40 kt add 500 ft. 41-50 kt add 1000 ft. 51-60 kt add 1500 ft. >60 kt add 2000 ft.

Terrain and Wind Influence on Pressure Levels (con’t)

Temperature correction for OCA(H) figures Pressure altimeters are calibrated to indicate true altitude under

ISA conditions. Any deviation from ISA will result in erroneous readings.

When temperatures are less than ISA an aircraft will be lower than the altimeter reading.

The error is proportional to the difference between actual and ISA temperature, and the vertical distance of the aircraft above the altimeter setting datum, i.e. height above touchdown. The error is approximately 4ft/1000ft for each °C of difference (1% per 2.5°C).

To ensure adequate obstacle clearance on approach add figure in body of table (see handout) to calculated DH/MDH.

In general if a table is not given then use the 4% per 10°C rule to calculate the height correction

In practice nobody worries about this correction. It is mainly an item for theoretical examinations.