There’s a familiar list of announcements when boarding a plane, but what do they actually mean? Our undercover pilot shines a light on pre-flight procedures.

I’m sure you will all have heard announcements such as “doors to automatic and cross check” or “the cabin lights will shortly be dimmed for take-off”. But do you know what is actually happening or why? In this column, I’m going to explain a bit more about some of these pre-flight procedures.

When an aircraft departs from the gate, you will hear the announcement: “Cabin crew doors to automatic and cross check.” You may have noticed that this prompts the cabin crew to operate a lever on each of the doors. This “arms” the door, meaning that should it be necessary to evacuate the aircraft in an emergency, the escape slide will automatically inflate once the door is opened. Cross check simply means cabin crew examine each other’s work by switching sides to check the levers on the other side of the plane.

Similarly, when the aircraft arrives at the gate you will hear: “Cabin crew doors to manual and cross check”, which means they will place the lever in the disarm position. Now, when the door is opened, the slide will not inflate and cause injury to the ground staff meeting the flight. It has happened!

Speaking of doors, many people have asked me if it’s possible to open a door during the flight. Sorry, Hollywood, but the answer is no, for two reasons. Firstly, aircraft doors are mechanically locked as soon as they are closed before take-off. Secondly, the door is designed to move inwards before opening outwards, and when the aircraft is pressurised at altitude, the force exerted is far too great, making it physically impossible to open.

Another question I am often asked is why do we dim the lights before take-off and landing – particularly when it’s dark outside? It’s not to help you sleep! There is a condition called nyctalopia, more commonly known as night blindness. This is where you suddenly go from a brightly lit environment to a dark one and suffer temporary blindness as a result, which could be the case if you had to evacuate an aircraft at night. Dimming the lights in the cabin allows your eyes to adjust and prevents this from happening.

Mobile mayhem

Another topic that raises much discussion is the requirement to activate flight mode on your mobile devices and whether it really makes a difference. From my experience, I can say without a doubt that it does. Many years ago, I was a first officer on a Boeing 747-200 flying to the US. On this older type of Jumbo we had the benefit of a flight engineer onboard, who sat behind the pilots and controlled the different aircraft systems. We were mid-Atlantic when he noticed that the aircraft was starting to depressurise. Fortunately, he had previous experience and concluded it was due to someone attempting to use a mobile phone.

The explanation for this is rather scientific. Radio waves belong to a type of wave known as electromagnetic waves. By their nature they can induce an electric current in a system leading to an unwanted response, which in this case led to the aircraft starting to depressurise. I made an announcement to the passengers to ensure their devices were in flight mode and shortly after the problem rectified itself without further incident.

Radio waves emitted by mobile devices are also known to cause interference with aircraft navigation and communication systems that also use radio waves. This is particularly critical during the approach and landing, which require precise navigational accuracy. Often passengers will say they left their phone on by mistake and it didn’t seem to matter. This may well be true, but if everyone were to do the same then this could potentially cause problems. Bear in mind that for most of the flight your device will not be able to find a signal, but will continually search for one, thereby draining your battery.

On the topic of batteries, these can also present problems. You will hear cabin crew announce that should you lose your mobile device down the side of the seat then don’t move the seat, but ask a member of the crew to assist. Lithium batteries do not like to be crushed or have pressure exerted on them as this can cause a condition called “thermal runaway” to occur, where the battery will become increasingly hot and eventually an explosive ignition will occur. The resulting fire can be intense and difficult to extinguish. So to sum up, all these rituals and procedures are there for a very good reason. Now you know why!