What is the Density Altitude?
The Density Altitude describes the altitude at which an aircraft "feels" it is operating due to the current air density - regardless of the actual altitude. It is a Relevant power factorwhich results from the pressure altitude, temperature and humidity. The warmer, the thinner the air - the higher the Density Altitude.
Example: An aerodrome in Switzerland at 1400 feet MSL can behave on a hot summer day with 32 °C and QNH 1013 hPa like 4500 feet or more "feel". This means that the aircraft behaves in terms of performance as if it were taking off from a high mountain airfield - with all the consequences.
Effects on engine, lift and controls
1. reduced engine power
Piston engines deliver less power in hot, thin air. Turbo engines and turboprops are also affected, albeit less dramatically. The propeller has less "bite", which prolongs acceleration and reduces the rate of climb.
2. longer starting distance
As less thrust and lift are generated, the rolling distance is significantly longer. This can be further exacerbated on grass tracks or damp surfaces.
3. higher true airspeed with the same IAS
The IAS (Indicated Airspeed) remains the same, but the actual speed over ground (TAS) is higher in thin air. This means more kinetic energy - both during take-off and landing.
4. longer landing distance
The higher approach TAS not only increases the touchdown point, but also extends the braking distance. In addition, the braking effect of the air (aerodynamically) is reduced. This is particularly critical on short or high-altitude runways such as Samedan (LSZS), Courchevel or Zell am See.
5. limited tax effect
Control surfaces (elevator, rudder, ailerons) lose effectiveness at low air density. This can lead to delayed or less precise reactions, especially when touching down or during go-around.
Typical mistakes - and how to avoid them
- "That's enough" mentalityMany pilots underestimate the effect of high density altitude. A 30 % longer take-off distance can mean the "end" for an aircraft with already limited power reserves.
- Incorrect power calculationPilots who rely solely on the altitude or outside temperature are often mistaken. One Calculation of the Density Altitude is part of every summer flight preparation.
- Loading not observedOverloading becomes even more critical at high density altitude. If you can take off in normal conditions, you may be able to take off with the same load in hot weather. no more off.
Recommendations for flight preparation
- Study performance dataThe flight manuals of many aircraft contain take-off and landing distance diagrams as a function of temperature and altitude. These should always before the flight.
- Use online calculators or appsTools such as ForeFlight, SkyDemon, Garmin Pilot or AvPlan offer integrated density-altitude calculators.
- Adjust flight time and locationFlying early in the morning or in the evening reduces the problem. Alternate airfields with longer runways can also be an option.
- Install safety margin50-100 % safety reserve for take-off and landing are recommended in midsummer - especially on grass or gravel tracks.
- Optional: Check rescue systemsSome overall rescue systems (e.g. CAPS) have restrictions on altitude or speed - pilots should know the operating limits.
Special attention in the Alps
With its valleys, high plateaus and thermally active slopes, Switzerland is regarded as a Complex flying area. When high temperatures are added to the mix, the density altitudes often rise to over 6000 feet - even on places like Zweisimmen, Grenchen or Locarno. Climbing performance of only 300 ft/min at MTOW are not uncommon.
This topic is therefore regularly covered during familiarisation flights in the Alps - but even experienced pilots should never underestimate the effects of the summer heat.
Further reading & links
If you would like to delve deeper into the subject, you will find valuable information in the following articles and sources:
- "Summertime: high heat, low power and the right flight tactics" (MeteoSwiss)
- "Summertime = High-Density-Altitude-Time" (AOPA Safety Publications)
- "Flying in the heat: Density Altitude and the 'Backside of the Power Curve'"
- FAA Advisory Circular AC 61-84 (USA): High Density Altitude Operations
- BFU - Summer flight safety tip (Germany)
Conclusion: summer flights need a cool head
High temperatures are not an exception, but have long been part of the flying season in Central Europe. The Density Altitude is one of the most important factors influencing safety - it affects performance, controllability, route requirements and decision-making behaviour. Whoever integrated into flight planning at an early stagewho prepares for scenarios and acts consciously will fly safely even in midsummer.
Because as is so often the case here too: The weather doesn't make the difference - the pilot does.
Source references:
MeteoSwiss