SafetySense leaflet 7, “Aeroplane performance”, available like all such leaflets free for download from
draws attention to the reduction in acceleration on take-off, and also to the additional ground run required to stop after landing if the surface of the runway is short wet grass. Many pilots have discovered that the increase of up to 60% quoted is no exaggeration. If longer grass is wet, there is also the potential for the propeller to spray water onto the wings during the taxi and take-off phases. Certain aircraft types are known to suffer a notable loss of performance with wet wings. However, as the pilot of a Musketeer apparently found recently, the hazards of wet grass are not restricted to the decrease in take-off and landing performance. It seems that having gathered speed during a downhill taxi, when the pilot attempted to slow down and turn on the wet grass, the aircraft slid sideways into a hedge, causing damage to the wing, fuselage and tail. It is important to consider the effects on our flight of all the environmental conditions we are likely to experience on a particular day, and make appropriate allowances.
We were recently reminded when flying from a local grass field just how limiting wet grass that’s a touch long can be. We watched the farmer spend a couple of hours cutting the taxyway grass but he completely ignored the runway! Before TO we walked part of the runway to see the state of the grass, I wouldn’t have wanted it to have been any longer and any wetter and as it was our departure wasn’t achieved with loads of room to spare and proved that the time we spent calculating everything carefully with the inclusion of recommended safety factors was time well spent. The two bits we didn’t get right however were that, the farmer said the grass was OK when we phoned for PPO but on inspection after landing it was longer than it should have been, in my book anyway, and the other often forgotten trap was that the grass was DEW LADEN and it stayed like that all through the day. If we had done what many pilots do and not allow any factoring I think our departure would have been very interesting and a touch ‘agricultural’ to say the least.
Always consider that its not just getting into the field that counts, you generally have to get out again too and that will take more distance. If you do not believe me consider the Air India Constellation that landed at RAF Northolt instead of Heathrow, by mistake, great landing but they had to take it out in several pieces on the back of a couple of lorries!
For information on factoring see the CAA Safety Sense Leaflet
Always consider: Work out the factored calculation to get out before you consider going in(unless you are going to leave the aircraft there for good). If you can get out on the runway you will get in on it(unless conditions change in the meantime or there are slope issues)