ALTIMETRY

INTERNATIONAL STANDARD ATMOSPHERE the average conditions of the atmosphere in a temperate climate surface temperature 15 deg. C surface pressure 29.92” Hg (1013.25 hPa) lapse rate 1.98 °C/1000 ft tropopause height ~ 36,000 feet tropopause temperature -56.5 deg. C stratospheric isothermal to ~ 65,000 feet

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

PROBLEMS WITH THE ISA Sea Level pressure is not always 29.92” Hg (1013.25 hPa) Sea level temperature is not always 15 °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

SURFACE PRESSURE ERRORS

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!

TEMPERATURE ERRORS

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!

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

ALTIMETER SETTING

ALTITUDE VARIATIONS AND ALTIMETER SETTINGS

DRIFT AND ALTIMETER ERROR

CORRECTIONS FOR PRESSURE ERRORS Find out the surface pressure below In ISA the surface pressure is 29.92” Hg or 1013.25 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!

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!

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%

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.

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

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

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

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

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

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%

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

MORE SIMPLY PUT

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

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 20 2 21 to 25 3 > 25 4

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

DENSITY ALTITUDE CORRECTION Temperature difference between -1 °C and 36 °C is 37 degrees. Density correction is 120 x 37 = 4440 ft which is added to the airport elevation The density altitude is 8000 + 4440 = 12,400 feet

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

EFFECTS OF DENSITY ALTITUDE

OOPS!!!