The difference between pressure and density altitude - every pilot learns it. Not having a good command of this formula means having all the more to deal with - and preparing for the worst. The higher the temperatures, the more critical this becomes.
The danger lies in the combination of heat and altitude (“high density altitude”).
Article in AeroRevue 09/2008 - Oliver Baer
Low pressure, high temperatures and an airfield at high altitude: This combination can mean the start of a perilous adventure, unless you are well acquainted with the performance data of your aircraft. For example, when taking off in a Piper Archer PA-28-181 from an airfield in the Swiss midland region at a temperature of 30°C, the distance before take-off increases by around 100 metres in comparison to a situation with a standard temperature (1,500 ft 12°C) - and the aircraft will not reach an altitude of 50 ft until 150 metres further on. If the same calculation is applied to a mountain airfield, the additional altitude has an even greater effect, and can present a significant level of danger if the pilot is ill-informed. This is why every pilot in training learns that it is imperative to calculate the take-off distance as part of the pre-flight preparations.
Pressure and density altitude
A Lycoming engine (O-360-A4M) burns a mixture of air and fuel in a ratio of 15 to 1. If the air density, and thus the density of the proportion of air in the mixture, changes (even with an optimal mix setting), this affects the performance of the engine. There are two factors which affect air density:
- Air pressure
High air pressure creates high density, as the air is being compressed. With an increase in altitude, however, the air pressure decreases (the weight of the air column on top of it reduces), as does the density, and, consequently, the performance of the engine. With a rise in temperature, the air expands due to an increase in kinetic energy, which also leads to a reduction in density whilst at the same time diminishing the engine's performance. So how much of an influence do these two parameters have on the take-off distance? There are two terms which are of relevance here: pressure and density altitude. Pressure altitude is simply the runway height but with the additional factor of the prevailing air pressure (QNH). An air pressure that is below the standard (1013.25 hPa) results in a higher pressure altitude than the runway height, namely an additional 28 ft per hPa deviation. The density altitude is the pressure altitude adjusted for non-standard temperature. If the current airfield temperature is higher than the ISA (International Standard Atmosphere) temperature, then 120 ft must be added per °C. A higher temperature thus results in a significant rise in the density altitude. The engine performance reduces - performing as it would at the (theoretical) density altitude.
Good flight preparation is an absolute must
The tables and charts commonly used nowadays are hugely useful in simplifying the take-off distance calculation. Nevertheless, a knowledge of the QNH and the airfield temperature at the planned take-off time are important prerequisites for a reliable result. So how does one obtain these two parameters? The QNH and temperature can be read off an up-to-date METAR, if the take-off takes place shortly thereafter and it is assumed that the values will not change significantly in that time. If there is no METAR available for the airfield in question, the METAR for a nearby airfield can be used as an indication.
Example of a METAR with current temperature and pressure values:
LSZH 221350Z 26009KT 230V320 CAVOK 31/14 Q1018 NOSIG=
On a hot summer's day, the pilot needs to be aware that the temperature will change significantly throughout the course of the day. A number of bulletins contain forecast values for these parameters, which have a much more marked effect on performance than changes in air pressure:
1. The forecast for engine-powered aviation, produced twice a day, contains temperature warnings if these are predicted to rise above 30°. "Hazards: In the lowlands, temperatures above 30 degrees in some areas."
2. Temperature forecasts are published several times a day in the TAFs for the Zurich, Geneva and Samedan airports: 230300Z 230413 VRB03KT CAVOK T25/08Z T29/11Z= (interpreted as: temperatures to reach 25°C at 08:00 hours UTC, 29°C at 11:00 hours UTC).
3. In addition, pilots can access information on the forecast temperatures at any given airfield from the MeteoSwiss personal meteorological advisory service for aviation (0900 162 737, 3 CHF + 1.50 CHF per minute).
Minimising the risk of misjudgement
In addition to the considerable lengthening of the take-off distance when there is a high density altitude, there is also a huge reduction in the climb rate. When taking off from a high-altitude airstrip, it must be taken into account that the rate of climb will be compromised. The following pointers are useful in countering the problems of reduced performance due to altitude and temperature:
- the weight is reduced, either by not having the full capacity of passengers on board, and/or by taking on less fuel (this entails a new take-off distance calculation)
- the mixer is set to the optimal fuel-air mixture which will enable the maximum possible, albeit reduced, performance to be achieved (only to be used for mountain airstrips, and in accordance with the manufacturer's guidelines)
- after take-off, the flight tactics are such that the aircraft can climb over flat ground and fly horizontally to cross over rising terrain (particularly important when crossing passes in the Alps).
When armed with a knowledge of this set of problems along with thorough pre-flight preparation, stress-free flights can be undertaken even on hot summer's days and from mountain airfields, whilst the risk of a misjudgement of the aircraft's performance is significantly reduced.
Airfield altitude: 1,500 ft, QNH: 1,006 hPa, temperature: 30°C
1. Calculation of ISA temperature at airfield altitude:
15°C – (1.5 x 2°C) = 12°C ISA: at sea level: 15°C; reduction by 2°C per 1,000 ft in altitude
2. Calculation of pressure altitude:
1,500 ft + (7.25 x 28 ft) = 1,703 ft; Difference between prevailing QNH and standard pressure x 28 ft (here: 1,013.25 hPa - 1,006 hPa = 7.25 hPa)
3. Calculation of density altitude:
1,703 ft + (18 x 120 ft) = 3,863 ft; Difference between prevailing temperature and ISA standard temperature x 120 ft (here: 30°C - 12°C = 18°C)