Monday, June 11, 2012

June 11th, 2012

Fresh Air Matters... with Capt. Yaw

Sadly, the past week has been full of news of aircraft crashes. May the souls of the departed rest in peace and may those who are charged with investigation be given the wisdom, patience and discernment to carry out effective, timely and accurate reporting.

The overshoot at Kotoka and the flight into buildings in Lagos has shaken West Africa and caused a number of reactions – some good, some not so good, others simply knee jerk or ill informed.

The common factor in both accidents was ‘Nigerian Registration’. Pretty much everything else is different. Therefore, trying to point at ‘Nigerian-isms’ as the root cause is ill advised. As my mother used to say ‘careful what you say, since there but for the grace of God go I’.

The differences are stark between the two accidents. One was a cargo plane coming off the end of the runway after touch-down, in rough weather, in Ghana; the other a passenger plane, with a declared emergency, on approach, in clear weather, colliding with buildings, as the crew struggled with reported engine issues, in Nigeria.

One accident is about braking and stopping and distances, the other is about lack of, thrust and a rate/angle of descent below the prescribed path. One apparently had a West African crew at the controls, the other, reportedly an American pilot and Indian co-pilot at the helm.

Historically, Ghana has an excellent safety record – amongst the best in the world. Nigeria has a historically poor record, yet notably the past five years have shown marked improvements, and the prestigious issue of an FAA top rating, which does not come lightly. Ghana is hot on the heels to win the same FAA accolade.

Do such accidents only happen in Africa? One would think so if you read the reports. However, just last year I witnessed a fighter aircraft overrun a runway, nearly ploughing into thousands of people and other aircraft on the ground. In December 2009 an American Airlines nearly new Boeing 737, with around 150 people on board, overshot the runway in Jamaica, the aircraft breaking into three pieces, in an, apparently, somewhat similar set of circumstances to the accident in Accra.

All over the world accidents happen. They do not call for ‘dramatic reactions’, they call for ‘measured responses’. As I have mentioned in this column before, ‘every checklist item, every rule and every regulation is the result of somebody having died’.

I was asked in interview ‘what caused the accidents?’, to which the answer is a clear ‘we must wait for the results of investigating committees’. Such investigations take months, and the outcomes are never as clear as the victims and their families want, anywhere in the world, but they at least point us in the right direction.

With regards to the accident in Ghana, all that we can say with certainty, at this point, is that ‘weather was a contributing factor’. That particular aircraft flew regularly into and out of Accra – it was not as if it was not an experienced operation in West Africa. We can also note that an aircraft landed safely before, and after, the Allied Air cargo plane. Around the time of the accident it was raining heavily and there has been talk of possible ‘aquaplaning’, the term for when a tyre moves through and on top of (ie breaking contact with the hard surface) a thickness of surface water remaining on the tarmac which then creates a skidding like effect, preventing frictional braking by contact with the tarmac surface.

Whether or not aquaplaning was the cause, part cause or absolutely nothing to do with the accident – which is the job of the investigating committee to determine, it is worth understanding the term that has come up in so many reports related to the accident. The calculation for potential aquaplaning is a simple one, on a surface with standing water (it need not be very thick, even less than 1mm can create the effect) ‘potentially experienced when the speed in knots exceeds nine times the square root of the tyre pressure in psi’.

Let us consider your car for a moment, even though the formula is a little different for cars. Assume a tyre pressure of 30psi. The aquaplaning speed will be 9 x √30 or 49kts which is about 91km/hr. In reality, the car tyre has a tread that works to eject the water, and provided you have a good tread on your car, drive on suitable road surfaces and drive responsibly, aquaplaning is rarely an issue. However, should you lack good tread, have low pressure, be over-speeding, on a badly drained, low-friction road surface and/or have other factors at play, you could find yourself aquaplaning. However, in aircraft the tyres are very different and much the ‘water clearing’ is built into the tarmac runways surface (by grooves and camber) to international standards. This is achieved in the runway structure and is incredibly effective – hence we rarely hear of aquaplaning! Assuming a tyre pressure on the aircraft of 64psi, the aquaplane speed could occur at around 9 x √(64) or 72knots or 133km/hr. This speed is lower than the touchdown speed of many larger aircraft; therefore, in certain unique circumstances it is possible to aquaplane on touchdown. Thicker water layers and other factors will play into the equation, and the above are only ‘rule of thumb’ pilot calculations. One thing to note is that a locked tyre (one that has stopped spinning and the brake is still on) will continue to aquaplane down to around 7.7x√(tyre pressure in psi), or in the above case to 54kts or 99kph.

Other factors that can, and will, play into such an accident are ‘touch-down point’. If the aircraft touches down further into the runway, there is less runway left to roll-out on, and an increased chance of overshooting the runway. The same with coming in at a higher speed, or a tail-wind, poor brakes, tyre conditions and other factors that affect the landing-roll distance even on a good day with no rain.

The aircraft that landed in Accra was 727, which, on a still day, with dry surface, a density altitude of 2000feet (assuming temps of around 30C and 100% humidity) and all other things being equal, would normally land in around 1600m, and, according to the available data on the Boeing site, in a little under 2000m on a wet runway ( - look at the last diagrams).

Kotoka has a LDA (Landing Distance Available) of about 3000m when landing on runway 21. Therefore, it is clear that there is more at play here than just weather, but that the weather had a part to play in the accident.

With all of the science in the world, and all the facts that can be gleaned, we will never know the EXACT cause of the accident, but many of the contributing factors will come out, and it will make flying into Accra in inclement weather less hazardous for all, for that is the way aviation works.

What struck me most about this accident is the behaviour of our services and the wonderful people of Ghana. Despite the challenges of the weather, the emergency services and associated personnel, all appear to have handled this incident with deft professionalism that puts Ghana in a positive light in regards to reaction time and approach. By contrast, the accident in Nigeria resulted in complicated scenarios as they sought water to extinguish the flames, and experienced difficulties in accessing and securing the site.

Accidents do happen. It is how we react to it that sets us apart. Let us hope that the investigating comitee will come out with a detailed explanation of the causes, and recommendations that will be rapidly implemented by the authorities. In the meantime, it is clear that Ghana is an exceptional place with amazing people, seeking to grow as responsible citizens of Planet Earth and a safer, more robust, aviation industry for all to enjoy.

Capt. Yaw is Chief Flying Instructor and Chief Engineer at WAASPS, and lead Pilot with Medicine on the Move, Humanitarian Aviation Logistics ( e-mail

1 comment: