Originally Posted by
Hughes500
Gordy not a bad one really as the grass and ground absorbs some of the shock as the aircraft touches down, hence you see indents in the ground. You can't seriously lell me that landing on a hard surface has less of a shock to the aircraft ? Of course the skids splay out as this is the first " crumple zone " the manufacturer puts in to protect us. The skids still start to splay on grass, you just dont get the harder shock especially when a student gets it wrong and one has to recover it
I'm afraid you are totally missing the point, H500. It's not the vertical loads that are of concern. It is the horizontal component of the touchdown velocity that is of much greater importance. If there is any forward motion at touchdown and either a) the skids are not perfectly aligned and/or b) the turf is not perfectly smooth, this could result in a dynamic rollover. On pavement (asphalt, tarmac, whatever you call it in your neck of the woods) you can pretty much get away with murder since the surface is very smooth and level, with a coefficient of friction far below that of even a putting green. Sure, the grinding sounds are horrible. While those sounds, and the smell of burning carbide or metal, can be concerning, the reality is that the ability to slide safely is much, much greater than on grass. It is that which is important, not the vertical component.
Also, it's worth nothing that from a vertical perspective, it's not really that the grass is absorbing impact so much as that the grass surface is providing a different character to the ground effect as you touch down. Leaving aside any differences in ground effect from soft surfaces vs. hard surfaces, given the same vertical speed at impact a soft surface will keep the skids from spaying more so than on pavement, and this will transmit a higher shock load to the airframe.
The bottom line is that hard surfaces are safer. Period. Dot. Physics and all that sort of thing. Now if you want to argue that the additional safety margin afforded by hard surfaces is inconsequential certainly you are free to do so. But as Gordy has noted, there are far more touchdown auto related accidents on soft surfaces than on hard surfaces, regardless of how many you and others have survived unscathed. If you have hard surfaces available to you why would you not avail yourself of them to gain that extra margin of safety? Your instructor should be beating you about the head and shoulders for too much forward velocity, or vertical velocity, at touchdown regardless of what surface you have chosen. So choose the one with a more homogenous surface and a lower coefficient of friction, i.e. a hard surface, just in case things go a little sideways.