Low-G Conditions and Mast Bumping “G” is an abbreviation for acceleration due to the earth’s  gravity. A person standing on the ground or sitting in an  aircraft in level flight is experiencing one G. An aircraft in a  tight, banked turn with the pilot being pressed into the seat  is experiencing more than one G or high-G conditions. A  person beginning a downward ride in an elevator or riding  down a steep track on a roller coaster is experiencing less  than one G or low-G conditions. The best way for a pilot to  recognize low G is a weightless feeling similar to the start  of a downward elevator ride. Helicopters rely on positive G to provide much or all of their  response to pilot control inputs. The pilot uses the cyclic  to tilt the rotor disk, and, at one G, the rotor is producing  thrust equal to aircraft weight. The tilting of the thrust  vector provides a moment about the center of gravity to  pitch or roll the fuselage. In a low-G condition, the thrust  and consequently the control authority are greatly reduced.  Although their control ability is reduced, multi-bladed (three  or more blades) helicopters can generate some moment  about the fuselage independent of thrust due to the rotor  hub design with the blade attachment offset from the center  of rotation. However, helicopters with two-bladed teetering  rotors rely entirely on the tilt of the thrust vector for control.  Therefore, low-G conditions can be catastrophic for two- bladed helicopters. At lower speeds, such as initiation of a takeoff from hover  or the traditional recovery from vortex ring state, forward  cyclic maneuvers do not cause low G and are safe to perform.  However, an abrupt forward cyclic input or pushover in  a two-bladed helicopter can be dangerous and must be  avoided, particularly at higher speeds. During a pushover  from moderate or high airspeed, as the helicopter noses over,  it enters a low-G condition. Thrust is reduced, and the pilot  has lost control of fuselage attitude but may not immediately  realize it. Tail rotor thrust or other aerodynamic factors will  often induce a roll. The pilot still has control of the rotor disk,  and may instinctively try to correct the roll, but the fuselage  does not respond due to the lack of thrust. If the fuselage is  rolling right, and the pilot puts in left cyclic to correct, the  combination of fuselage angle to the right and rotor disk  angle to the left becomes quite large and may exceed the  clearances built into the rotor hub. This results in the hub  contacting the rotor mast, which is known as mast bumping.  [Figure 11-8]  Low-G mast bumping has been the cause of  numerous military and civilian fatal accidents. It was initially  encountered during nap-of-the-earth flying, a very low- altitude tactical flight technique used by the military where  3.  Always use a two-step lift-off. Pull in just enough  collective pitch control to be light on the skids  or landing wheels and feel for equilibrium, then  gently lift the helicopter into the air. 4.  Hover high enough to have adequate skid or landing  wheel clearance from any obstacles when practicing  hovering maneuvers close to the ground, especially  when practicing sideways or rearward flight. 5.  Remember that when the wind is coming from  the upslope direction, less lateral cyclic control is  available. 6.  Avoid tailwind conditions when conducting slope  operations. 7.  Remember that less lateral cyclic control is available  due to the translating tendency of the tail rotor when  the left skid or landing wheel is upslope. (This is true  for counterclockwise rotor disks.) 8.  Keep in mind that the lateral cyclic requirement changes  when passengers or cargo are loaded or unloaded. 9.  Be aware that if the helicopter utilizes interconnecting  fuel lines that allow fuel to automatically transfer from  one side of the helicopter to the other, the gravitational  flow of fuel to the downslope tank could change the  CG, resulting in a different amount of cyclic control  application to obtain the same lateral result. 10. Do not allow the cyclic limits to be reached. If the  cyclic control limit is reached, further lowering of the  collective may cause mast bumping. If this occurs,  return to a hover and select a landing point with a  lesser degree of slope. 11. During a takeoff from a slope, begin by leveling the  main rotor disk with the horizon or very slightly into  the slope to ensure vertical lift and only enough lateral  thrust to prevent sliding on the slope. If the upslope  skid or wheel starts to leave the ground before the  downslope skid or wheel, smoothly and gently lower  the collective and check to see if the downslope skid or  wheel is caught on something. Under these conditions,  vertical ascent is the only acceptable method of lift-off. 12. Be aware that dynamic rollover can be experienced  during flight operations on a floating platform if the  platform is pitching/rolling while attempting to land  or takeoff. Generally, the pilot operating on floating  platforms (barges, ships, etc.) observes a cycle of seven  during which the waves increase and then decrease to  a minimum. It is that time of minimum wave motion  that the pilot needs to use for the moment of landing  or takeoff on floating platforms. Pilots operating from  floating platforms should also exercise great caution  concerning cranes, masts, nearby boats (tugs) and nets. 11-14