Aircraft Accidents and the Landing Circle

Articles in several aviation magazines drew attention to a funded project called the Endless Runway:

It is a circular runway, three kilometers (1.86 miles) in diameter with a circumference of 9.42 kilometers (5.85 miles); it encircles the entire airport, taxiways and terminals.  Access for trains and vehicle traffic is through a tunnel system under the runway enclosure.

It’s definitely a twenty-first century concept; I find the idea – in my opinion – to be incredibly simple in its design while being reckless in its execution.  Don’t get me wrong, the runway probably does what its designers say it will: there is an economy of real estate; good utilization of approach/departure despite which way the wind blows; and an avoidance of rerouting to alternate airports due to crosswinds.  I’m not a pilot, so my concerns aren’t directed at the pros and cons experienced by flight operations.

I’m a mechanic, and think as such.

Thirty-five years ago, I drove a tall Grumman courier truck between Mineola, NY, and Newark-Liberty airport, roughly forty miles; this particular truck was designed for short, low speed delivery routes, not long haul, interstate drives; it was used to transfer late freight that missed the container transport vehicle.  The Grumman was loaded as expeditiously as possible, meaning the vehicle became top-heavy.  With a narrow wheelbase, the truck was very hard to handle when changing lanes or making turns because of the high center of gravity.

I watched the proposal video showing an A380 capturing the runway and beginning its rollout.  If anyone has had the pleasure of being in the cockpit as your airliner lands, it’s interesting.  During high arrival/departure times, the pilots execute a Hi-Speed taxi; this is where the aircraft lands and must expedite to the nearest taxiway to clear the runway for arriving/departing traffic; it’s not landing speed, but it’s fast.  Under the best conditions, you can feel the entire fuselage fight the desire to keep going forward while the nose gear turns right or left.  My experience with Hi-speed taxis have been on narrow body aircraft, e.g. 727, 737 and DC-9, airliners that are low to the ground.  Most older narrow-bodies react more favorably to these forces than one with a high center of gravity, e.g. 747, A380 or any aircraft with a high gear profile accounting for wing mounted engines, which is … just about any airliner these days.  Wide-body aircraft can weigh in excess of 500,000 pounds, landing at 130 knots or more; that’s a lot of inertia to account for.

Add weather in the form of snow, rain and/or ice, and the forces acting on the aircraft should be a reminder of why aircraft rollout in a straight line.  During these weather conditions aircraft employ anti-skid on a level runway to keep from sliding sideways; this means, on the main gear trucks, a wheel that ‘locks’ due to sliding on ice is hydraulically ‘released’ along with its opposing wheel to prevent a skid.  This device may not work as efficiently on a circular runway that centrifugal force acts to move outward.  The designer has engineered an inward angle to the runway, which forms centripetal forces toward the center; this may nullify or exaggerate the condition.  It also raises another issue: wing strikes, which I’ll address shortly.

Again, I’m a mechanic, so let’s look at the effects on the aircraft.  An aircraft is designed with side load stresses built into the landing gear.  I’ve swapped out landing gear on 727 and DC-10 aircraft; the gaps built into the various joints and rotating mounts are minimal, as thin as a .001 inch of gap.  There is no slop in a landing gear’s mount in aircraft that normally move in linear motion, straight ahead.  However, introduce consistent side loads where centripetal and centrifugal forces act against the straight line; stresses not designed into the aircraft may not be redesigned in an affordable or safe manner.

A top heavy fuselage, e.g. a cargo airliner, wants to move in the opposite direction as the turn; let’s not forget Sir Isaac Newton’s first law of motion: “… An object in motion continues in motion with the same speed and in the same direction, unless acted upon by an unbalanced force.”  The ‘unbalanced force’ is often the gear and the tires, each wanting to follow the curve of the circular runway.  All tires, whether the airliner has six or eighteen, will also have to be modified to withstand the enormous side loads.  The results of these loads may not be evident immediately, but will surface over time and, perhaps, catastrophically.

I can’t get any information on the runway’s in-sloping angle, but best guess is it’s about a fifteen to twenty degree slope taken from the animation.  My second concern deals with wing strikes; the video shows the right main gear, if touching down wings level, would allow the left wing and winglet to strike the runway.  The video shows the A380 banking on approach and touching down on a matching side angle of fifteen to twenty degrees.  The departing flight matches the incline until its wings are clear of the runway and level flight can be achieved.  But airliners are designed to wings level landings; they may hit the runway at a bank and/or nose left/right of center while fighting a crosswind, but the manufacturer’s design is what the onboard computers operate under.  In my mechanical opinion, it is too easy for a wing or outboard wing-mounted engine to come within striking distance.  Entire landing procedures, from a programming standpoint, would have to be rethought and redesigned for each airliner.

And then there’s one last point: Run off.  Runways are designed to drain different forms of contamination, e.g. snow, de-icing fluid, away to the sides; this is to encourage run off on a 262-foot wide runway, away from the centerline.  Rain can cause hydroplaning; its ability to flow easily would allow quick run off on a runway that is angled fifteen to twenty degrees, assuming, however, that the temperature remains above freezing.

The circular runway requires water to travel more than twice the distance to one side, allowing for the extra width – as much as twice the width of a normal international runway of 262 feet – built in for centripetal forces.  Glycol and propylene glycol are used in de-icing and anti-icing fluids; they are more viscous – resistant to flow; this allows them to stay on the wings during taxi to prevent ice/snow build up; they are shed during the take-off roll as the aircraft gains speed.  Where the de/anti-ice fluids end up are on the runway.  This would prove to be a traction safety concern for both landing and departing aircraft if the fluids can’t be regularly removed to an acceptable limit in between flights.

Urea is also a concern; where glycol products are used to de/anti-ice aircraft, urea, and like substances, are used to prevent snow/ice build-up on the runway’s surface.  Again, this needs to run off naturally or be removed frequently.  Urea can affect the safety of arriving/departing aircraft, but also can affect the integrity of the aircraft itself; its damaging effects are a concern to the aircraft’s structure and exposed areas.

From an accident investigator’s point of view, I see as-of-yet unimagined errors that can happen to man and machine.  I hope the issues can be worked out; the design is interesting and probably will benefit the aviation industry.  But its simple concept may prove more difficult to overcome than we are led to believe.

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