A setting that allows you to have a helicopter serviced on a heliport.
It has always been very annoying to not be able to use helicopters to their full merit. They are slow, break down a lot, become obsolete pretty fast, and for servicing have to visit a hangar at an airport. This poses great logistical problems, since you either have to build a "Small Airport" nearby, if it is not already obsolete (you can enable it with the "Always allow small airport" patch) or send the helicopter to some far-away airport that will surely screw with its schedule.
With this patch enabled, a helicopter landing on a heliport will automatically be serviced by the skilled technicians stationed there. This will hopefully greatly improve their performance. Please note that this also applies to airports that have dedicated helicopter landing spots, helipads, such as the International Airport, or an Oil rig.
This feature is enabled by default. You can disable this patch through the configure patches -> stations window or use the following syntax in Openttd.cfg:
[patches] serviceathelipad = false