The Cessna 208 Airspeed Indicator

With jump flying, understanding ground speed is essential for determining exit separation. This speed is usually obtained from the Satellite Navigation unit and represents True Ground Speed, which should be compared only to True Airspeed (TAS), not Indicated Airspeed (IAS).

True speeds are derived by measuring the time to travel between two known points. IAS, displayed on the airspeed indicator, is calibrated based on air pressure entering the pitot tube. Influenced by atmospheric conditions such as air density, IAS does not adjust for altitude or temperature variations and measures the aerodynamic forces on the aircraft’s surfaces.

The air is thinner (less dense) at higher altitudes, meaning less air flows over the aircraft's wings per unit of time compared to lower altitudes for the same speed. To compensate for this reduced air density and maintain the necessary lift (airflow over the wings), an aircraft must increase its true airspeed (TAS).

However, the indicated airspeed (IAS), which the pilot sees on the airspeed indicator, doesn't show this increase in true airspeed. IAS is calibrated based on standard atmosphere conditions at sea level, and it reflects the dynamic pressure (the pressure of the air molecules hitting the aircraft) rather than the actual speed through the air. As the aircraft ascends and the air becomes less dense, the same dynamic pressure corresponds to a higher true airspeed. Therefore, even though the aircraft needs to fly faster regarding TAS to achieve the same aerodynamic effect (like lift), the IAS will show a lower speed than TAS because the dynamic pressure impacting the pitot tube is reduced.

Environmental factors like temperature and pressure impact air density and aircraft performance. Temperature and pressure typically decrease as altitude increases, influencing air density and airspeed accuracy.

A rule of thumb for calculating true airspeed with altitude is to add 2% to the IAS for every 1,000 feet of elevation. For instance, climbing at an IAS of 100 knots to 10,000 feet results in a TAS of 120 knots. In the UK, it is rare to experience zero wind at this altitude, but it occurs. This often leads to misconceptions of having a 20-knot tailwind when the ground speed reads 120 knots, sometimes confusing when jump runs are reversed, and the ground speed remains unchanged.

The C208 Airspeed Indicator includes a circular slide rule, similar to a mini CRP-5 "whizz wheel." The outer scale shows outside air temperature, and the inner movable slide indicates altitude in feet.

The referenced image shows the aircraft at 12,000 feet with an outside temperature of 0°C, resulting in a TAS of 120 knots on the outer ring. The slide rule is configured to a standard pressure setting of 1013.2 hPa (29.92 inHg).

Given that this gauge is set for pressure altitude, it should also be considered a rule of thumb as a practical guideline. While these methods usually provide TAS readings within a 3 to 5-knot margin of accuracy in the cold UK, discrepancies may be more pronounced in regions with extreme temperature variations.

In summary, if you are skydiving from 15,000 feet and wind conditions are less than 10 knots at altitude, the aircraft will have a high ground speed. If the separation time between groups is 6 seconds, you should exit the aircraft 6 seconds after the last person in the previous group leaves. This means you should be exiting directly 6 seconds later, not waiting 6 seconds, then spending 4 seconds climbing out, 3 seconds geeking the camera, and another 3 seconds giving a count. With the aircraft covering 0.2 miles (1056 feet/322m) every 6 seconds, 16 seconds is half a mile, and in no wind, the last ones out will struggle, aka land off.