Definition
A potentially catastrophic condition in two-bladed teetering-rotor helicopters in which the rotor hub strikes the rotor mast (the vertical shaft the rotor sits on). It typically results from low-G or near-zero-G flight conditions that allow the rotor disc to tilt excessively relative to the mast, often leading to mast separation and in-flight breakup.
Plain English
The rotor hub slams into the shaft it is mounted on, which can break the shaft and cause the rotor to separate from the helicopter. It is most likely when the pilot pushes the cyclic forward in a way that unloads the rotor.
Context Anchor
Seen in helicopter aerodynamics, operating limitations, and accident discussions, especially for helicopters with two-blade teetering rotor systems.
Derivation
From 'mast,' meaning the vertical shaft supporting the rotor (borrowed from the nautical term for a ship's vertical pole), and 'bumping,' the everyday word for a sharp contact between two objects. The name describes the event literally: the rotor hub bumping the mast.
Why Pilots Care
It can cause immediate loss of rotor control and separation of the rotor system in teetering-head helicopters.
Analogy
Picture a seesaw mounted on a center post. If one side is forced too far, the seesaw can hit the post. In mast bumping, the rotor hub can hit the helicopter’s mast in a much more serious and damaging way.
Grounding Statement
The key idea is that the rotor system can move far enough to strike its own supporting mast, and that contact can be catastrophic.
Intuition Check
Do not treat “bumping” as a small or harmless contact here. Mast bumping means dangerous rotor-to-mast contact that can cause the helicopter to break up or become uncontrollable.
Example Sentence 1
The flight instructor warned the student that pushing the cyclic forward abruptly in a Robinson R22 could unload the rotor and lead to mast bumping.
Example Sentence 2
After the low-G push-over the instructor demonstrated the correct recovery to prevent mast bumping.