- A cabin crew member to her/his chief: The securing strap for the oxygen bottle at 3R is loose.
- Chief to pilot: The 3R oxygen bottle cannot be secured, something is wrong with the strap.
- Pilot to maintenance over the radio: We need someone onboard, we have an issue with the oxygen bottle strap at 3R.
- Maintenance guy arrives, after checking it, he goes to the pilot: it will take 5 minutes to repair it and another 5 to get it, I just radioed the guys to bring the spare part.
- Captain over PA: Dear passengers, we have a 10 minutes advisory due to a small technical problem. However, the weather is blah blah..
- 10 minutes later, the maintenance guy goes to the pilot: Captain, the guys couldn't find the spare part, we need to get it from the main spare parts shop at the far side of the airport, it will take 10 more minutes.
- Captain over the PA again: Dear passengers, it seems that we need 15 more minutes, blah blah..
- etc. etc.
The above is real world scenario, which I faced hundreds of times during my years working as a cabin crew.
Even if a similar thing happened before passenger boarding had started, the pilots would give green light for it to start because they would expect the plane to be fixed and they would have better chances for the plane to take-off on-time. It’s better than delaying the boarding while they fix the problem, which virtually guarantees causing a delay. This is the general logic if they think there is a good chance of fixing the plane on-gate.
Airlines do not lie, they just like to think about the "best possible scenario" when passing information about the delays.
As for "swapping airplanes", this is really not something airlines do easily:
- They wouldn't have an aircraft that has nothing to do and waiting for "swapping".
- Even if they wanted to, usually this only happens at "bases" or main hubs for the airline.
- Also, when swapping the planes, you will need to swap the catering and other stuff which is specific for that flight, that's not easy and it's costly.
- What about luggage and cargo? what if they already started loading that?
- Lastly, swapping the plane will usually cause two delays instead of one, the first delay is for the original scheduled flight, and a second delay for the flight in which the plane was pulled from. so why create two problems instead of one?
If there's some big technical problem or the pilots decided that the plane is grounded, the airline then will consider swapping the plane. New airlines' systems are smart enough to suggest the best scenario: should the flight be cancelled or should the airline swap the plane with another flight (For example, one that has a scheduled, non-mandatory check-ups that can be re-scheduled)? This will usually cause a chain of delays, which can be neutralized after a few flights for the same plane (making up a few minutes from each flight until finally it catches up to its original schedule).