- Serious malfunctions on modern commercial jets exceptionally rare
- Traffic controller admits human errors occur on "daily basis," often due to language barrier
- Technology helps avert errors, but most important is trust between pilots and controllers
As authorities continue to search for the Malaysia Airlines jet that went missing on March 8, thoughts turn to air safety.
It's exceptionally rare for planes to experience malfunctions, especially serious ones. "Planes don't fall out of the sky at 36,000 feet," says CNN's aviation correspondent Richard Quest.
Even when errors do occur, most often human in nature, airlines and traffic controllers employ a vast array of procedures to ensure our safety.
Hong Kong Airlines was involved in nine incidents in which pilots apparently disregarded instructions from air traffic controllers (ATCs), including a plane taxiing onto a runway without permission and failure to follow instructions about altitude and direction.
Hong Kong's Civil Aviation Department is still investigating those incidents.
Other recent cases of miscommunication between pilots and ground controllers include:
• July 2010: The captain of an Air Blue flight disregards instructions from traffic control and crashes into mountains near Islamabad, killing 152.
• June 2013: Two Boeing 747s narrowly miss colliding over Scotland when one plane turns right and the other left -- effectively doing the opposite of ATC instructions.
• December 2013: A British Airways jumbo jet crashes into a building at Johannesburg airport when the pilot goes down the wrong taxiway.
According to Ady Dolan, an air traffic controller at London Heathrow Airport who spoke with CNN for this story, human errors between pilots and air traffic controllers occur on "a daily basis."
But while common, most errors go unnoticed and are of no threat to safety, thanks to established systems of communication and technology.
English ... but whose English?
According to Dolan, controllers at Heathrow deal with 85 airlines and 1,350 flights a day.
Controllers need to be able to communicate with pilots of many different nationalities, he says.
English is the language of aviation and vital for pilot-controller communication.
"We're lucky that English is the language of the air," says Dolan. "If English is not the pilot's first language and they only come to Heathrow occasionally, we need to afford extra care to that pilot.
"We can't speak with speed and abbreviation as we would to someone who comes here several times a day."
A pilot who often flies to China and Southeast Asia, and who spoke with CNN on condition of anonymity, says pilots and air traffic controllers generally enjoy a good relationship, especially in Hong Kong where ATC standards are high.
"But China can be a bit of an issue," he says. "We should all be speaking English, but for a lot of people it's their second language."
The pilot says airlines have varying policies on recruiting pilots with good English, and this can cause problems.
Patter and chatter
The International Civil Aviation Organization (ICAO), a United Nations agency that sets and regulates standards of air safety, says standardized phraseology is an important part of pilot-controller dialogue.
For example, before the 1977 KLM-Pan Am crash, the pilot reportedly told traffic control, "We are now at takeoff" as he moved down the runway.
The controller understood this to mean the plane was ready to take off, but was still stationary and waiting for further instructions.
For this reason the controller didn't warn the pilot about another plane on the runway, which was obscured by thick fog.
"Pilot and air traffic controller radiotelephony communication is an important part of training leading up to licensing for both functions," says an ICAO spokesperson. "ICAO guidance requires that both controllers and pilots use standardized phraseology in their communication exchanges.
"ICAO is continuously reviewing and updating its standardized phraseology guidelines to better meet the needs of the air transport system and ICAO language proficiency requirements have been in place since March 2008."
The relationship between controllers and pilots functions well generally, says South African Airways pilot Sarah Jones, who flies across South Africa and Africa.
"There are lots of structures in place to try and prevent miscommunication between us. Obviously, there has to be trust," she says.
While Jones says ATC standards are high in airports such as OR Tambo in Johannesburg, they're not as good at some other African airports.
She says it can be challenging when controllers and other pilots don't speak English.
"You have to be very vigilant, listening to the other traffic and having good situational awareness," she says. "It's an issue when they are speaking, say, French or Brazilian, and you can't understand what other pilots are telling ATC in airports where you would be more naturally cautious anyway."
Human error inevitable
Errors that occur as a result of pilot-controller misunderstandings are normal, but go largely unnoticed by passengers, according to the pilots and controllers interviewed for this story.
As air traffic increases, so does the potential for poor communication.
That's why it's important to have solid backup procedures in place, says Heathrow air traffic controller Ady Dolan.
Dolan works for UK-based NATS, a provider of air traffic services in the UK and more than 30 other countries.
The company also provides strategies for dealing with potential problems.
"At any airport if you have humans involved there are going to be errors," he says. "Our job as an ATC provider is to spot the potential for that error before it takes place and then when it does happen to have mitigations in place to correct that."
He says typical errors might be "simple," like a plane ending up facing south instead of north on a stand.
"Safety has not been compromised, but you as an ATC need to have in mind the aircraft may end up facing south instead of north. It's the kind of human error that takes place on a daily basis; it has no impact (but) is not noticed."
The pilot who flies to China and Southeast Asia says the nature of flying and human beings means "dozens" of small mistakes are made regularly.
"There are mistakes, but we are there to manage it and that's where standards of training and checking and a rigorous operating procedure come in," he says. "It means you can get on the deck with a pilot you have never flown with before.
"It's only a minor risk, but there is always the potential for things to go wrong and to escalate into a position where things could be harmful.
"That's why it's positive that cases like the ones in Hong Kong have come to light and are being investigated.
"As far as ATC goes, if they tell you to descend to a certain level, one pilot sets the level and the other gets the reading and repeats it and then the other repeats it again. It's a system of double checking.
"That doesn't mean we never have 'altitude busts' [aircraft descending to altitudes for which they aren't given clearance] but there is very good backup with Traffic Collision Avoidance System and Ground Proximity Warning System."
Sarah Jones agrees on the potential for errors.
"If ATC gives you a climb instruction and a heading instruction you could get those confused," she explains. "That's why you always have one pilot flying and one monitoring.
"Mistakes can happen regularly but they get corrected quickly. If there is any possible confusion you always double check with ATC quickly, that's the system."
Electronic communication replacing human contact
The daily process of handling weather, delays, spacing aircraft evenly for landing and takeoffs and squeezing maximum capacity for runways is getting more complicated.
Sarah Jones says that the balancing act between safety and keeping traffic moving is maintained by mutual respect.
ICAO also believes the potential for things to go wrong can be handled.
"The role of effective communication has long been considered a key component of safety," says the spokesperson. "ICAO has dedicated and continues to dedicate much effort and resources to ensuring our guidance in this area is comprehensive and aligned with operational needs."
The increasing use of technology over human voices to communicate with the flight deck offers one solution.
For example, Heathrow conveys electronic clearances for takeoff directly to the flight deck. Even so, it's unlikely we'll see computers completely taking over traffic control in this generation.
"You've got to be able to trust the information that is being passed to you or the whole system falls apart," says Dolan. "If the pilot is in any doubt as to the safety of the instructions and is having to second guess things you know you are in a bad situation."