A Medical Helicopter was questioned by ATC about departing a hospital pad without first contacting ATC as required by an existing LOA. The reporter was unaware of the subject LOA.

Narrative

When departing airport area hospitals it is SOP to contact the Tower as soon as possible as the hospitals are within the Class C airspace. Because the hospitals are at a lower elevation than the airport communications; in most cases; are not possible while on the ground. The procedure has been to take off and contact the Tower as soon as communications are possible to get clearance through the airspace. I departed a hospital not being able to contact ATC from the pad. I kept trying as I climbed out and established communications with the Tower within about sixty seconds. They were busy so it took about thirty seconds longer than normal to get a transmission to them. The Tower cleared me to proceed VFR at or below 3000ft on course. Everything was pretty consistent with normal operations in the area. Shortly after getting my clearance; the Tower called me and asked me to call them when I landed about a possible deviation. When I landed; I called the Tower and was informed that I had violated a Letter of Agreement which requires; when departing any of the hospitals in the area; that the pilots not take off from the pads until communication with the Tower was established by radio or telephone. The Controller I talked to also indicated that this is a program wide problem. I had never heard this. It was not in my local area training and orientation. I immediately called the aviation program manager who informed me that he was not aware of any such Letter of Agreement. I also talked to my lead pilot who also indicated that he was not familiar with that Letter of Agreement. There is no copy of the LOA at our base and a subsequent discussion with other pilots has revealed that none of them are familiar with it either.Suggest we just need to make sure that everyone is aware of the proper procedures for dealing with this. If there are no definitive procedures we need to come up with them and get everyone on the same page. It is a difficult situation because of the communication problems. An alternate frequency has been established for contacting the Tower but it has not solved the problem. Maybe the resolution lies within establishing VFR routes out of town that will not conflict with traffic from the airport even though there is no initial communication with the Tower. It is a very short time that we are out of contact.