Pilot handbook: How high altitude may down aircraft in seconds

The airborne chopper with Governor Ole Tunai and his bodyguard aboard before the crash. [Peter Muiruri, Standard]

A 58-second long video footage showing a green chopper struggling to fly before crashing to the ground emerged online on Saturday a few hours after a chopper crashed at Ildamati, Narok County.

From the clip, the chopper can be seen scanning for a landing spot before tumbling to the ground head-fast at the tip of thick wheat field leaving a trail of dust before curious onlookers rushed to the aircraft to rescue the victims.

Kenyans would then learn that those who were aboard the Karen Blixen Camp Trust-owned chopper were Narok Governor Samuel ole Tunai and his bodyguard Emmanuel Mugalu.

According to the police statement, the chopper that carried Tunai who had attended a burial in the area developed a mechanical hitch at the tail leading to the crash.

The occupants, who had no visible injuries, were taken to Narok Referral Hospital for checkups.

“We have learnt of the disconcerting incident of the involvement of Narok Governor Samuel Tunai in a chopper crash in Melili, Narok County. We wish to inform members of the public that Tunai survived the crash and is currently receiving medical attention at Narok Referral Hospital,” Council of Governors chair Governor Wycliffe Oparanya said in a statement released by the Council after the crash.

But moments later, Marc Goss, the pilot who flew the ill-fated chopper talked to Standard Digital to clarify the possible triggers of the incident.

Mr Goss leads the Rapid Response Unit at Mara Elephant Project and has earned a lucrative reputation for driving elephants away from human settlements. He attributed the crash to the unconducive weather and high altitude. But from the footage, the sky was clear and the weather was serene, hinting at alternative cause for the crash.

High altitude risks

Standard Digital looks at how high altitude could pose serious risks to the stability of airborne choppers.

In a handbook titled Helicopter Flying Handbook,  America's Federal Aviation Administration (FAA) details the technical protocols for flying choppers and possible perils of human errors and natural calamities.

In Chapter Two, titled: “Helicopter Emergencies and Hazards”, the agency advises that high altitude can cause mechanical issues in various components of the chopper.

It partly states: “At higher altitudes where the air is thinner, tail rotor thrust and efficiency are reduced. Because of the high-density altitude, power plants may be much slower to respond to power changes.

When operating at high altitudes and high gross weights, especially while hovering, the tail rotor thrust may not be sufficient to maintain directional control, and LTE can occur.”

To further understand what the tail rotor is all about and the importance of its thrust, Nasa explains in its website that the tail rotor serves two purposes.

“It provides a counteracting force to the helicopter's main rotor; without the sideways thrust produced by the tail rotor, the torque generated by the main rotor would spin the helicopter's body in the opposite direction,” Nasa notes on its website.

Thus, with high altitude reducing the efficiency of the tail rotor, the aircraft loses balance for maintaining unidirectional movement due to lack of a mechanism to counter the force from the blades (main rotor). All these may lead to Loss of tail rotor effectiveness (LTE).

The LTE is the unpredictable change of rotation of the blades, which may trigger loss of direction and result in a crash.

“It is very important for pilots to understand that LTE is caused by an aerodynamic interaction between the main rotor and tail rotor and not caused by a mechanical failure,” FAA explains in the handbook.

Adding, “Some helicopter types are more likely to encounter LTE due to the normal certification thrust produced by having a tail rotor that, although meeting certification standards, is not always able to produce the additional thrust demanded by the pilot.”