Quote:
Originally Posted by Rodolphe
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If the Gunner concludes to exit his flying Defiant through the bottom hatch, the Pilot need to lower the landing gear as this configuration automatically retracts the rear VHF aerial mast into the aero plane belly.
That done the mast won't spear the Gunner as he, after moving the aerial wire aside, escapes through the bottom hatch, . 
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From the posts in the link it seems that the lower exit route was more difficult to use. If the plane is flipped over on take off or landing though, it would become essential.