Water Ballast, High Density Altitude, and a Short Strip
You plan a 300 km task from a plateau site at 1400 m elevation. The temperature is 30 °C, QNH 1005, giving a density altitude well above the field elevation. Your glider has a maximum take-off mass of 600 kg, an empty mass of 350 kg, and a wing loading of 50 kg/m² at maximum mass. You intend to carry 120 kg of water ballast. The only usable runway is 900 m of dry grass, with a 1.5% upslope and a 5 kt tailwind component forecast at launch time. The tow plane is a 180 hp aircraft with a placarded maximum aerotow speed of 130 km/h. Your polar shows a best glide ratio of 42:1 at 50 kg/m² and 38:1 at 60 kg/m².
1. Evaluate whether to launch with full ballast, and quantify the trade-offs.
Read the worked answer
Launching with 120 kg of ballast from a 900 m upslope grass strip with a tailwind and high density altitude is not supportable. The trade-offs:
- Take-off distance: density altitude reduces engine power and propeller thrust on the tug and reduces the glider's lift at a given speed, so the ground run and the distance to 15 m both increase. A 1.5% upslope and a 5 kt tailwind both add to the ground run. A 900 m strip with these factors leaves little margin, and the tailwind component alone can add substantially to the distance.
- Climb performance: the tug's climb rate will be reduced, and the glider's sink rate at the higher wing loading is higher, so the aerotow will be slower to reach a safe height.
- Glide performance: the polar shows only a 4-point improvement in glide ratio (42:1 to 38:1) for a 10 kg/m² increase in wing loading. That gain is small compared with the take-off and climb penalties.
- Handling: higher wing loading raises stall speed and reduces thermalling margin at low altitude.
The correct decision is to launch with reduced or no ballast, or to wait for the tailwind to drop. If ballast is carried, it should be jettisonable and the jettison procedure briefed before take-off.
2. If you do launch with reduced ballast, how would you plan the task and the return, and what would you monitor?
Read the worked answer
With reduced ballast, the plan should be built around the density altitude and the afternoon wind.
- Task length: shorten the task or set a turnpoint that keeps you within easy final glide of the field in the expected conditions. Use the actual polar for the ballast carried, not the full-ballast polar.
- Final glide: calculate the final glide using the forecast wind and the expected sink, and add a safety margin. At high density altitude the true airspeed for a given indicated airspeed is higher, so the still-air glide ratio is unchanged but the time and fuel-equivalent margins change.
- Height bands: set a minimum working height and a decision height for abandoning the task. On a plateau, the terrain around the field may be higher than the field, so the decision height must account for terrain, not just the field elevation.
- Monitor: surface wind and gust spread, cloud base, and the actual climb rates compared with the forecast. If the wind increases or the lift weakens, return early rather than pressing on.
- Ballast: keep the jettison area clear and briefed, and dump before landing if the strip is short or the wind is strong.
The judgement tested is matching the aircraft configuration to the day's performance, not to the task's ambition.