With its armed police and dog patrols, Manchester airport exudes a reassuring sense of security that befits the country’s third largest aviation hub. The reality is worryingly different, as I discovered earlier this year while posing as a security officer for a BBC documentary.

Over a three-month period of checking passengers for weapons, searching bags and patrolling the airport, I saw metal detectors that didn’t work and planes left unlocked and unattended overnight. After the 9/11 terror attacks,many airports increased their investment in security but Manchester went in the opposite direction, cutting its security budget by £8m and its workforce by a third. Although there have since been some moves in the opposite direction, I wanted to know how safe the place was now. So I applied for a job as a security officer.

The interview was straightforward and I was offered the position. After two-and-a-half weeks’ training, I began my new life as a £13,500-a-year aviation security officer.

It was clear from the outset that one of the primary defences against terrorists, the metal detectors, were sometimes faulty. I had been apprehensive about filming at the airport, fearful about how I was going to smuggle my hidden camera through this apparently sophisticated security system. Incredibly this never proved to be a problem. I walked through three different metal detectors wearing my filming equipment and it triggered the alarm only once.

On another occasion a manager conducted a test by walking through the arch carrying an 8in butterfly knife, a kind of flick knife, which didn’t activate the alarm either. The machine was closed down and reset; meanwhile the manager went on to test the second metal detector in the room, this time armed with a large pair of scissors. This machine was also faulty — yet passengers were still allowed to use it before boarding planes.

The proper procedure when someone set off an alarm was a time-consuming ritual involving a clothes search with a hand-held “wand” detector, another trip through the metal detector and possibly a private search. Meanwhile, queues built up and security officers were feeling the pressure to hustle passengers through.

In our training we were taught that a fundamental part of our job was to conduct random bag searches. A set ratio of these searches was laid down. In one part of the airport we were required to record these searches. On numerous occasions I saw these entries being falsified. Sometimes they “searched” empty trays and counted them as bags.

One of the most disturbing security lapses concerned parked aircraft at night. The airport apron was patrolled throughout the night. If a plane was seen to be unlocked, the correct routine was to notify the airline, wait for someone to arrive and lock it, before fixing a paper seal on the door. A security officer told me that sometimes they would drive past open planes to avoid this hassle.

Another colleague told me that once a member of the public had got on to the apron and had been found sleeping in an open plane.

I decided to go and have a look for myself. Of the 15 planes I inspected, two had no paper seals, 12 had steps leading up to them (against regulations) and one was open and unguarded. I was able to film inside, uninterrupted.

The apron brought together three factors that could lead to a terrorist disaster — an inadequate perimeter fence, ill-maintained patrols and open planes. It seemed incredible that nobody could see the danger.

To keep security staff on their toes, covert tests were conducted by various agencies, including the Department for Transport. The idea was that inspectors, dressed as ordinary passengers, would trip a metal detector and we were supposed to find their concealed weapons.

Although these tests were meant to be secret, I saw staff being tipped off. On several occasions I was given detailed descriptions of inspectors, down to their clothing and the weapon they were carrying. I saw a team member phoning a colleague with a description of an inspector who had just visited us.

Bizarrely, security staff were offered a £60 bonus if they found a weapon in one of these covert tests but nothing if they discovered a real illegal weapon. It showed an odd sense of priorities.

The problem is not that specific individuals are falling short and ought to take the blame, it is the environment in which they work.

Conscious that the BBC has a responsibility to ensure we don’t reveal anything that could help terrorists, we gave the airport ample notice of our findings so that any necessary changes could be implemented before Whistleblower is broadcast on Tuesday night on BBC1. Clearly, Manchester airport did not learn the lessons of September 11. I pray that it has now.

Michelle Cox was talking to Stuart Wavell