Although hangar doors are not explicitly mentioned in the EASA Part 145, the environmental requirements for a hangar are, according to the EASA, not so easy to be met without doors.
That is why, in its document UG.CAO.00024 (UG stands for User Guide), the EASA identifies the doors as the mean to meet the facility requirements for hangars intended to host heavy maintenance of aircraft.
There are however some exceptions and here is the golden rule that you should use in order to determine whether your hangar needs to be fitted with doors or not:
Such a requirement for hangar doors is meant to meet the intents of the EASA facility requirements 145.A.25 and any specific condition coming from the TCH (i.e. Boeing, Airbus, etc).
Such TCH conditions should be identified in the AMM (Aircraft Maintenance Manual) and may refer to situations like painting, weighing or jacking up of the aircraft.
In any case, while the UG.CAO.00024 is just an “User Guide”, supposed to guide the user to the correct application of the requirements, the actual mandatory facility requirements are instead pointed out in the 145.A.25 as follow:
- Facility should ensure protection from weather elements (145.A.25a),
…and (as per 145.A.25-c-1):
- temperatures must be maintained such that personnel can carry out required tasks without undue discomfort,
- Dust and any other airborne contamination are kept to a minimum and not be permitted to reach a level in the work task area where visible aircraft/component surface contamination is evident. Where dust/other airborne contamination results in visible surface contamination, all susceptible systems are sealed until acceptable conditions are re-established,
- Noise shall not distract personnel from carrying out inspection tasks. Where it is impractical to control the noise source, such personnel are provided with the necessary personal equipment to stop excessive noise causing distraction during inspection tasks,
- Where a particular maintenance task requires the application of specific environmental conditions different to the foregoing, then such conditions are observed. Specific conditions are identified in the maintenance data.
We have to assume that the above written conditions are those that apparently convinced the EASA that the easiest way to meet them is through hangars fitted with doors.
However, the EASA recognised that appropriate procedures may be put in place in order to operate hangars without doors, which is the reason why it accepted those hangars without doors already approved before the 2014 (the year when the decision for the doors was taken) and did not make the requirement retroactive.
Therefore, according to my understanding , any MRO able to demonstrate that the above written requirements (above written bullet points) can be met, should be able to get an exemption from the UG.CAO.00024, though I am not sure about the chance of success for such a possibility.
UG.CAO.00024 can be found here.