Carrying capacity is understood essentially as the load any living or non-living body can bear safely. An obese person is certainly not comfortable carrying excess weight. It can either be due to body chemistry or, as commonly believed, due to negligent living and eating habits. But ‘Miss Moti-vation’ (the cartoonist for the Nepali Times), taking a bold touch, coaxes those labelled ‘moti’ to enjoy what they are, without being embarrassed or experiencing self-pity. Being very bubbly and active, perhaps she cannot be labelled as too moti after all. Not wishing to get entangled with the obese ornot obese argument, this piece aims to play it safe by dealing only with non-living things.

The more feet the better

As with our feet, the weight of a building is transferred on to the soil through its footings. The bearing capacity of the soil becomes critical from this perspective. The same principle guides the design of roads or airport pavements as well. The load of a vehicle is transferred on to the pavement through tyres. Naturally, a higher number of tyres help distribute the load to an even larger tyre contact area, reducing the pressure on the pavement. For the same load, a 10-wheeler becomes better than a truck with just 6. We often hear about our roads having to bear higher loads, resulting in their quicker deterioration. If so, this is primarily due to negligence on the part of the concerned agency which lets heavier vehicles to ply anywhere and everywhere in the first place. But the critical question is: do our roads get “built” as “designed”, or do over-laden vehicles get used as scapegoats, camouflaging nefarious practices? 

Like a truck, an aircraft, transfers its load on to the tarmac on landing. There is a misconception about big aircrafts being harmful to runways as they tend to be heavier. As with trucks, landing gear comes in different formats. Even a narrow body NAC B757 has 10 wheels in five bogie assemblies (one bogie has two wheels). These are arranged with two bogies each (four wheels) on either side as main landing gears (MLG). Erstwhile, Indian Airlines (IA) had ordered 5 bogie A320s in 1988, to access airports with bad runways. IA became the only airline to have five bogie versions. A higher number of bogies not only made the aircraft heavier, but it also a little expensive and thus, it did not earn favour.

And then there’s the A380, the whale jet. It comes with 22 wheels, with five gear positions instead of three. The aft side has two sets of landing gears for either side in two and three separate bogie combinations. The giant USSR built transporter, the AN-225 Mriya, incorporates a 32-wheel landing gear system (as shown in the picture). It has two bogies under the nose (not one as most) and 14 MLG bogies, seven per side. Mriya’s landing gears barely jut out of the belly unlike with the A380/B777. Another innovative difference is the way three bogie MLGs get tilted up, in the direction of flight, while extended. This allows the last two tyres in the three bogie assembly to make the first ground contact with more wheels gradually following as it settles down.

Strange combinations

It is said that there is nothing on earth or in the air quite like the AN-225. Being one and half times heavier than a fully loaded version of its biggest American counterpart, the C-5A, it has a massive takeoff weight of 600 metric tonnes. Even with a wingspan of nearly 90 metres and its huge bulk, it is extremely manoeuvrable. This is because 16 wheels, of the 28 in the MLG, are steerable! Besides that, it can carry a single piece of cargo with a volume of about 5500 cubic metres, which can weigh up to 200 metric tonnes. But its maximum cargo capability is 50 metric tonnes on top of that. 

Talking about strange wheel combinations, no aircraft can beat the American spy plane U-2 with just two landing gears in bi-cycle configuration. As such, it was unable to stand level without external support and it always landed with one wing rubbing against the runway. That apart, it could not land without two chase vehicles racing alongside, assisting the pilot with the last few feet of height reading. And in order to remain horizontal it required two temporary supports on either side of the wings. In a way, it was a system that appears far from perfect for a spy plane that is supposed to fly undetected over enemy territory. Surprisingly, India, despite not being in the pro-American camp, had covertly allowed the CIA to base its U-2s at Charbatia (Orissa) around 1963.    

We often hear of damage at Kathmandu airport caused by heavy planes. This often results in embarrassing airport closures, even during peak operational hours, so temporary repairs can be done. Hard pavement atop aging asphalt is widely believed to be the cause. Fundamentally, it is a misnomer to blame the ‘heavies’ alone. With a small contact area, the tyres of non-heavies can be as punishing to the runways. The runway foundation needs redoing by removing the damaged portions and substituting them with stronger pavement. Let us only hope that this work gets done, and gets done as soon as possible. And without any controversy as with the airport improvement programme that is currently cancelled.

Arjyal writes extensively on aviation