Martin served three years in prison after shooting Fred Barras, 16, at his home in Emneth Hungate, Norfolk, in 1999.
The National Transportation Safety Board said preliminary data showed conflicting readings about the altitudes of the plane and the helicopter.
The party will prepare for the May 1 local and mayoral elections with a campaign launch rally at the Arena Birmingham.
Martin served three years in prison for shooting Fred Barras, 16, at his home in Emneth Hungate, Norfolk, in 1999.