Why sealed runways are not always the safest
|Crumbling tarmac at Ilaga airstrip|
The trouble out here in the mountains of Papua is that almost all of the sealed runways are of pretty poor construction. Even before the tarmac is laid, the underlying surface is never fully levelled and smoothed off. This leaves a surface with lots of undulations on it allowing rain to collect in the dips and aircraft to be thrown about after touchdown. Then the base layer on top of that is usually too thin which after a few months of laden aircraft landing on it starts to subside even further, causing the undulations to be even more extreme. Then finally the top layer of tarmac is always too thin and so doesn’t take long to start to erode away and leaving behind large pot holes and cracks, ready to burst an unsuspecting aircraft’s tyres.
|Broken and bumpy tarmac at Jila airstrip|
|Bilogai airstrip’s crumbling tarmac before they replaced it|
Saying all that, it’s not like the unsealed runways are of any better construction, in-fact they’re mostly even worse bar a select few missionary airstrips. So what’s the big deal? Why do I consider sealed ones more dangerous? Well, it’s to do with the tailwheel configuration of the Porter’s undercarriage.
In all aircraft types, the centre of gravity is between the three sets of wheels. In a tailwheel aircraft the main wheels are at the front which puts the C of G behind them. In a tricycle undercarriage setup, the main wheels are behind the nose-wheel which puts the C of G is in-front of the main wheels. This is a crucial difference which drastically influences the controllability of an aircraft on the ground, especially when landing.
It is always important to land any aircraft dead straight, in the centre and in-line with the runway. After touchdown in a tricycle aircraft the nose-wheel is lowered and the aircraft will track straight in the direction you are pointed. You can then use that nose-wheel to help steer the aircraft during the landing roll and maintain a straight path along the runway’s centreline.
|Lovely smooth grass at Pagamba airstrip|
In a tailwheel aircraft such as the Porter, the same is true. You land on the mains and when the tailwheel touches the ground (usually all at the same time), you will track straight in the direction you are pointed. However, because the C of G is behind the main wheels any slight deviation from straight, and the tail of the aircraft will want to swing out further. To help circumvent this the tailwheel is always locked straight ahead for landing.
However, this means if you don’t touchdown dead straight, or you go over an undulation, the aircraft will spear off in that direction. This requires some quick work with the toe-brakes which are mounted on each rudder pedal and allow the pilot to apply the brakes to either the left or right main wheel. This individual braking action allows a swift adjustment to the yaw to bring things back straight. On an undulating runway, it can be a bit like doing a dance on the brake pedals to keep things straight.
Now, the main reason things are so much harder on a tarmac or sealed surface compared to a grass or dirt airstrip is due to the levels of grip available. Any aircraft on landing will have a certain amount of momentum which on the whole will carry you in a fairly straight line down a runway provided you’ve got the touchdown correct. On a grass or dirt airstrip, that momentum can actually help keep things straight, even if you hit undulations or didn’t quite touchdown straight because the wheels don’t have so much grip on the surface so can slide about a little. On tarmac, the wheels do have grip and pull the aircraft in which ever direction the three wheels are pointed. It’s this reason that most pilots learning to fly tailwheel aircraft will learn on a grass airfield before moving onto tarmac ones. It’s simply more forgiving.
|Lovely hard non-sealed surface at Gisilema airstrip|
So places like Ilaga with it’s broken tarmac, numerous undulations, potholes and faster landing speeds due to it’s 7500ft elevation actually do give me much more grief than somewhere like the newly constructed Gisilema airstrip with it’s 195m of smooth dirt. Just because an airstrip looks undramatic compared to another, doesn’t mean it’s any easier to land on!
|Current grass airstrip at Mapnduma with the new sealed one under construction (sad times!)|