It was truly shocking to see the tragic collision between two aircraft at Haneda airport yesterday, and of course amazing that the outcome was not so much worse. I was invited by the BBC World News program for a live interview to provide some context for this kind of accident and I thought afterwards that, whilst I wouldn’t want to speculate as to the causes of this particular event, it might be helpful for those unfamiliar with the topic, to put together a basic overview of runway incursion scenarios.

Historically, very few high speed collisions between aircraft have occurred on an active runway. Since two aircraft are involved, there is the potential for the worst of outcomes in terms of the number of lives lost. In every risk assessment I've ever seen for this type of accident, the assumption has been that all passengers and crew of both aircraft will be lost. The landmark accident in 1977, when a KLM Boeing 747 and a Pan Am Boeing 747 collided at high speed on the runway at Tenerife, remains the worst aviation accident in history, with 583 fatalities.

Having learned from accidents like this, and also from numerous near-miss (or as some say, near-hit) events and safety studies, the potential scenarios involved are by and large pretty well understood by the industry. For the sake of brevity I’ll just go over the most basic details from my perspective as a former airline pilot and air safety investigator.

What would be the most common way in which an accident like this could happen?

Without speculation as to what happened in this specific event, and in no particular order, here are the most common errors involved in setting up this kind of accident.

Air traffic controller error:

• Issuing an incorrect clearance to enter an active runway – this includes clearances to taxiing aircraft but also to landing aircraft.

Flight crew of the taxiing aircraft:

• The crew aren’t aware they are entering a live runway i.e. they have lost their situational awareness and aren't aware that they are entering the runway, or they might enter the wrong runway.
• The crew misunderstand an ATC transmission i.e. they believe that they have been cleared to enter the runway when they have not.
• Very unlikely but also possible, the taxiing aircraft is for some reason unable to stop at the runway holding point e.g. brake failure.

Flight crew of the landing aircraft:

• Landing without a clearance.
• Landing on the wrong runway.

A mix of ATC and flight crew actions:

• At a busy airport, spacing between arriving and departing aircraft is very important. This separation can be degraded quickly and for a variety of reasons e.g. if the departing aircraft is slow to commence the take-off whilst another aircraft is arriving.

What is the evacuation process in a situation like this? What is the evacuation process like when part of the aircraft is on fire? How long should it normally take for passengers to leave the aircraft?

Most of us will be familiar with what it’s like as a passenger on a normal flight in a full aircraft when it stops at the arrival gate - everyone gets up and wants to get off at the same time - all of a sudden the aisles are rammed full of people.

So we can imagine the potential for things to get chaotic in the cabin when it starts filling with acrid smoke and people are in fear for their lives.  Managing the process is obviously the key to a successful evacuation and airlines spend a lot of time training their flight and cabin crew on how to respond in this kind of scenario.

• The evacuation order is normally given by the captain once the aircraft has stopped and the engines have been shut down.
• The cabin crew will check if their doors are safe to open. The doors will already be in the ‘armed’ mode. This means that when the door is opened, the slide-raft should automatically inflate. Once that happens, the cabin crew member starts evacuating passengers through their exit.
• Sometimes an exit may be unsafe to use, for example if there are flames outside, in which case the cabin crew would block that exit and redirect passengers to a usable one, whilst continually reassessing their exit for any changes.
• When an aircraft comes to rest in an unusual attitude e.g. nose down, the use of some  evacuation slides can be a problem - those at the rear of the aircraft have a much steeper angle and it's more likely that passengers will be injured when using these. But when time is critical that’s frankly an acceptable outcome.
• By now the airport’s rescue and fire fighting services will be on the scene and, in addition to the obvious need to fight any fire, they will need to manage people on the ground, attending to the injured and assembling everyone in a safe area.
• The standard that has to be achieved during the certification of a new airline aircraft type is to demonstrate and evacuation of all passengers within 90 seconds with 50% of the exits unusable.

What is likely to happen next in terms of the investigation?

The Japanese Air Accident Investigation Commission will lead the investigation and representatives from all the stakeholders will participate e.g. the aircraft operators, the airport and air traffic management provider, even the aircraft manufacturers.

One of the first things investigators they will be doing is to gather evidence in order to establish a timeline or sequence of events. There are many sources of potential information.

• Interviews with the air traffic controllers, the flight crews, cabin crew and passengers, even the crew of other aircraft who may have been nearby.
• Reviewing the available recordings like the cockpit voice recorder and ATC communications recordings.
• Video footage and pictures.
• Inspecting the ground infrastructure e.g. lighting, markings etc.

The evidence will first help to establish the immediate cause of the accident i.e. the last thing to have gone wrong, or the last link in the accident chain if you like that analogy. But that is just the most basic aspect of the investigation process. In order to help prevent not only the same event from occurring, but also others where similar factors might come into play, investigators will spend a lot of effort analysing the preconditions for the event – there will be a mix of often complex reasons why the people involved took the actions they did on the day, and understanding those reasons is really at the heart of the investigation process.

I'm sure we will all be interested to learn from the investigation when its complete. In the meantime, looking at the footage of the A350 in flames, it does seem that some things must have gone right for the passengers and crew of the JAL aircraft to have survived, whilst at the same time mourning the loss of life on the Coast Guard aircraft.