1. ALTIMETRY

2. INTERNATIONAL STANDARD ATMOSPHERE
the average conditions of the atmosphere in a temperate climate
surface temperature deg. C
surface pressure ” Hg
( hPa)
lapse rate °C/1000 ft
tropopause height ~ 36,000 feet
tropopause temperature deg. C
stratospheric isothermal to ~ 65,000 feet

4. THE ALTIMETER
An aneroid barometer that indicates pressure as a specific altitude, generally
in feet
Calibrated using the ICAO International Standard Atmosphere (ISA)

5. PROBLEMS WITH THE ISA Sea Level pressure is not always
29.92” Hg ( hPa)
Sea level temperature is not always °C
The lapse rate is not always
1.98 °C/1000 feet
ISA assumes pressure decreases at a rate of 1” hg/1000 ft (1 hPa/30 feet). This is correct only in the lower levels

6. SURFACE PRESSURE ERRORS

7. PRESSURE ERRORS
If you are moving towards lower pressure without a revised altimeter setting - you are losing altitude
If you are moving towards higher pressure without a revised altimeter setting - you are gaining altitude
Hence the expression:
From High to Low, Look Out Below!

8. TEMPERATURE ERRORS

9. TEMPERATURE ERRORS
If you are moving towards colder temperatures without a revised altimeter setting - you are losing altitude
If you are moving towards warmer temperatures without a revised altimeter setting - you are gaining altitude
Hence the expression:
From High to Low, Look Out Below!

10. ALTIMETER SETTING
The pressure that when set on the altimeter will cause it to read the station elevation when the aircraft is on the ground. ( Indicated Altitude)
Used in the ‘Altimeter Setting Region’ below 18,000 feet. Above 18,000 feet use 29.92”Hg (Pressure Altitude)
Standard Pressure Region - use 29.92” Hg where frequent updated altimeter settings are not available

11. ALTIMETER SETTING

12. ALTITUDE VARIATIONS AND ALTIMETER SETTINGS

14. DRIFT AND ALTIMETER ERROR

15. CORRECTIONS FOR PRESSURE ERRORS
Find out the surface pressure below
In ISA the surface pressure is 29.92” Hg or hPa
Determine the difference from ISA
Correction is 30 feet/hPa or 1000 feet/1 in Hg
Add or subtract this correction value from the indicated altitude
Remember: if the pressure below is low; you are low so the sign must be negative!

16. CORRECTIONS FOR TEMPERATURE ERRORS
If the average temperature of the column beneath the aircraft is warmer than ISA, you will be higher than indicated.
If the average temperature of the column beneath the aircraft is colder than ISA, you will be lower than indicated.
From High to Low, Look Out Below!

17. CORRECTIONS FOR PRESSURE ERRORS
A temperature correction can be made if lapse rate is close to ISA:
4 feet per degree different from ISA per thousand feet AGL
Remember: if the temperature is colder; you are low so the sign must be negative!
winter inversion - calculate flight correction
and increase by 50%

18. COMBINED ERRORS
You can have both pressure and temperature errors. If so:
Step 1 - Correct the pressure error
Step 2 - Correct the temperature
error
Note: The altitude used in step 2 is the new altitude calculated in step 1. See the following example.

19. EXAMPLE OF COMBINED ERRORS
Consider an aircraft flying over 973 hPa low
at 10,000 ft indicated; OAT -25 °C. What is the
total error in altitude?
Step 1 - pressure error: pressure is too low by
40 hPa (1013 – 973 hPa)
correction is 30 ft x 40 = 1200 feet
New altitude= 10,000 – 1200 = 8800 ft
Closest 1000 ft altitude is 9,000 ft

20. EXAMPLE OF COMBINED ERRORS
Step 2 - temperature error
Calculate what the temperature should be at
9000 feet in the ISA
= ISA surface temp. – ISA lapse rate x height
(thousands unit only, i.e. use 22 for 22,000 feet)
= 15 – (1.98 x 9) = -2.8 °C (-3 °C rounded off)
OAT is -25 °C so it is 22 degrees C too cold – you
are going to be low
Correction = 4 ft x 22 deg x 9 = 792 ft

21. EXAMPLE OF COMBINED ERRORS
From an indicated altitude of 10,000
feet, the actual height is closer to:
= Indicated altitude – minus pressure correction – minus temperature correction
= 10,000 feet – 1200 feet – 792 feet
= 8008 feet

22. COLD AIR OPERATIONS There are times that using an altimeter setting
makes the situation even worse!

23. SOLUTIONS? Know the enemy! Be careful out there!
Other forms of altimetry - radar and GPS

24. DENSITY ALTITUDE
According to the NTSB, wind was cited as a cause/factor in over 76% of General Aviation weather-related accidents: Crosswind - 28% Gusts - 24% Tailwind, high wind, wind shifts - 8% each Low ceiling or fog - 18% High Density altitude - 8%

25. DENSITY ALTITUDE
Density altitude is the altitude in the ISA at which the air density would be equal to the actual air density at the place of observation. "Density Altitude" is the pressure altitude adjusted for non-standard temperature
Both an increase in temperature and humidity will cause a reduction in air density
In hot and humid conditions, the density altitude may be significantly higher than the true altitude

26. MORE SIMPLY PUT

27. CORRECTING DENSITY ALTITUDE
Correction is 120 feet times the temperature difference from ISA
the correction above assumes dry air
to correct for moisture you must use the virtual temperature

28. VIRTUAL TEMPERATURE
Virtual temperature - the temperature that dry air would have so that its density would be the same as the moist air
Virtual temperature correction is made using the dew point temperature and is always positive
dewpoint <10 deg. C add 1° to air temp.
11 to
21 to
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29. DENSITY ALTITUDE CORRECTION
You are at an airport at an elevation of 8,000 ft ASL. The air temperature is 36 °C. What is the density altitude?
What is the temperature error? In the ISA atmosphere the temperature at 8,000 feet should be 15 °C – (8 x 1.98 °C/1000 feet) = -1 °C

30. DENSITY ALTITUDE CORRECTION
Temperature difference between °C and 36 °C is 37 degrees.
Density correction is 120 x 37 = ft which is added to the airport elevation
The density altitude is = 12,400 feet

31. SIGNIFICANT PERFORMANCE PROBLEMS
TAKE-OFF WEIGHT
TAKE-OFF RUN
CLIMB PERFORMANCE
CLIMB RATIO

32. EFFECTS OF DENSITY ALTITUDE

33. OOPS!!!