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New Zealand pilots have been permanently grounded after testing revealed concerns about their medical fitness to fly, with some suspended because of depression, the aviation regulator has confirmed.
Commercial pilots are subject to rigorous psychological tests designed to weed out unstable individuals who pose a potential risk to passenger safety.
The Civil Aviation Authority said some pilots' licences had been suspended because of medical fitness - which can include psychological health concerns - though such cases were rare.
The monitoring process has numerous checks and balances, but an industry expert warns the system is not foolproof.
"Pilots like anyone else have problems in their life and that can affect their performance," New Zealand Airline Pilots Association technical officer Dave Reynolds said.
"People do go off the rails."
Authorities investigating the disappearance of Malaysia Airlines Flight 370 are probing the psychological backgrounds of pilots and crew.
A fellow pilot told the New Zealand Herald Captain Zaharie Ahmad Shah was in no state to fly the day the flight disappeared and could have taken the Boeing 777 for a "last joyride" after separating from his wife.
Mr Reynolds said Kiwi pilots operating commercial aircraft are regularly assessed for physical and mental health under a three-tier monitoring regime involving the Civil Aviation Authority, airlines and peer support.
Pilots are assessed at least once a year by specialist aviation doctors and also undergo psychometric tests to assess psychological fitness.
The doctors are legally bound to "flag up" any concerns to the regulator and airline employer, and a pilot's licence can be immediately revoked.
Pilots are also monitored in the cockpit by flight inspectors and undergo high-stress exercise scenarios every six months in flight simulators to test their ability - for instance trying to safely land a badly disabled jet.
"All commercial aircraft pilots will go through some form of psychological, psychometric testing. Internationally there are standards that are required to be met."
Asked if the system was rigorous enough to prevent a mentally unstable pilot taking control of an aircraft, Mr Reynolds said: "You can never be 100 per cent sure because ... one person can be perfectly fine one day and then go totally off the rails the next.
"A guy may go home and his wife says, 'I've been seeing the milkman, bye'.
"[But] outside that, yes, there are checks and balances in place to make sure, as reasonably as possible, that a pilot doesn't get into an aircraft who is psychologically unfit to do so."
Mr Reynolds did not believe Flight 370's disappearance was pilot suicide.
Air New Zealand declined to provide details of the airline's regime for monitoring pilots' psychological fitness, referring queries to CAA.
Anyone applying for a commercial pilot's licence is subject to a "fit and proper person" assessment by CAA.
It has powers to perform "total background" checks on pilots' criminal histories and physical or mental health problems.
Authority spokesman Mike Richards said this process had resulted in "adverse decisions", with some pilots' licences suspended or revoked, meaning they could no longer fly commercial aircraft. However, they could reapply at a later stage.
"But if we're not satisfied we can literally suspend someone's aviation documents almost on the spot and they just can't get on a flight deck.
"It does happen. The Director [of Civil Aviation] has far ranging administrative powers. We can pretty much prevent the wrong types of behaviour if we get any hint of it."
CAA manager personnel and flight training, John McKinlay, knew of pilots having their licences suspended because of depression.
"It is a risk and the risk has to be mitigated."
Background checks gave a person's history "which can be a reliable predictor of future behaviour."
"Our medical folk have got the ability to gain information from various sources if there is an issue that's going to affect flight safety."
- By Lane Nichols of APNZ