After a lengthy discussion, we have decided to update and clarify the motive rules for obstruction. This will hopefully serve two functions: 1) clarify the rules to all, and 2) relax some areas to make some shots easier to get accepted (especially ramp shots). We've tried to cover everything below in regards to obstruction, but if you think something has been missed, please let us know by posting here. As always, we retain the right to apply the rules as we see fit, and make exceptions when deemed necessary.
Motive Rules for Obstruction
RAMP VEHICLES (including tugs, loaders, baggage carts, etc...) are allowed to block part of the aircraft as long as they are:
 not blocking engines or wheels (exception: tugs are allowed to block engines/wheels if towing aircraft, and ground crew are allowed to block any part if not too prominent, which is at our discretion)
 below the window line
 part of active operations (this amendment is meant to exclude any aircraft parked with equipment in front of it. 'Active operations' is meant to include those aircraft actively loading or unloading.)
As was previously the case, for preserved subjects there is much more leeway when the obstruction is unavoidable, such as permanently affixed stairs, or fences.
Integral stairs on biz jets (and some larger aircraft such as 737s) are allowed to obstruct. Smaller crew ladders affixed to the sides of military aircraft are generally allowed, even on static subjects.
Jet ways are not allowed if blocking any part of the side of the aircraft facing the photographer.
For head-on shots, they are not allowed to block the engine/gear or a significant portion of aircraft if a wider view. Small blockage of the wing will be allowed for tighter crops.
Fences should not block any part of the aircraft. Fences taking up more than one-third of the frame can also be considered motive rejections even if not blocking part of the aircraft.
The terrain is allowed to block no more than 50% of the wheel. This does not apply to runway/taxiway markers, for which there is no allowable blockage as they are easily avoidable. For vegetation that is evenly spread out (i.e. not just one big clump of grass that could have been avoided, similar to a taxiway marker), there can be some tolerance past 50%, but this will be at our discretion.
Cones are ok if unavoidable (i.e. by shooting from different angle) and not obstructing too much. How much is 'too much' will again be at our discretion.
Shots with covers over a significant part of the aircraft will only be allowed if the aircraft is not on the db already. If we already have an image of it uncovered, then we will not accept it covered. This does not include engine/pitot covers. For partial/canopy covers, this again will be at our discretion.