1. Section 03 - Altimetry (cont) Lesson 09
Height Calculations
Minimum Safe Altitude
Flight Level
Transition Altitude
Transition Level
Transition Layer
Further Altimeter Calculations

2. Altimeter Barometric Error
Altimeter error due to changing surface barometric pressure

3. Terrain Clearance

4. 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.

5. 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.

6. Vertical Position References

7. 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.

8. 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.

9. 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.

10. Pressure Datums

11. QNH Less than SPS

12. QNH Greater than SPS

13. Terrain and Wind Influence on Pressure Levels

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

15. 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.