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DEPARTMENT OF FLYING SAFETY
THE UNITED STATES ARMY PRIMARY HELICOPTER CENTER
FORT WOLTERS, TEXAS
File No. 1-403-2
STUDENT HANDOUT
HELICOPTER ACCIDENT PREVENTION
"HOW TO CRASH A HELICOPTER"
b. Tree Landings
When a tree Landing is unavoidable or preferable, the pilot should select a touchdown spot based on the following considerations:
(I) The height of a tree is less critical than the height above the ground where it begins to branch. Tall trees with thin tops allow too much free-fall height after the aircraft passes through the branches.
(2) When dealing with young or short trees (twice helicopter height or less), the most densely and evenly wooded area should be chosen.
This is an ideal situation in which the bottom of the aircraft as well as the main rotor provide a cushioning effect.
(3) When dealing with large trees, resistance against the bottom aircraft structure should be avoided in such a manner that the fuselage and tail boom will settle between the tree tops before the main rotor engages the branches of the surrounding trees. In other words, the pilot should look for an area where the rotor disk meets equal resistance to tree top level with the "softest" spot for fuselage and tail boom to insure a tail-Low attitude at ground contact.
The general implication is that, although their branches may overlap, tree trunks should provide a clearance of at least 1 1/2 times the rotor diameter.
(4) If at all possible, main rotor contact with heavy trunks high above the ground should be avoided as it may result in loss of main rotor or transmission failure. If a retreating (metal) blade strikes very heavy lumber, the tendency of the transmission is to fail in the forward direction (with counter-clockwise rotor rotation,) The opposite is true when an advancing blade strikes a heavy obstacle, including the ground.
(5) A landing in a sparsely wooded area may require more finesse than landing in a dense forest canopy since the few individual trees act as obstacles rather than energy absorbers. Under these circumstances, the terrain itself will probably be the main touchdown area and hitting an obstacle prior to touchdown often leads to loss of aircraft control and an uncontrolled crash. For example, if the left side of the rotor disk were to settle into trees while the right side met no resistance, the afrcrait would tend to strike the ground on its right side.
(6) Brush-type Vegetation of less than helicopter height should be dealt with as if it were not there
(7) Clearings in woods should be treated with caution as they may contain tree stumps and other obstacles that may penetrate the aircraft's bottom.
(8) Dead trees are dangerous; they offer little energy absorption and tend to puncture the fuselage
(9) A tree landing should be executed with zero or near-zero ground speed and in a tail-low attitude If for some reason the pilot is unable to reduce forward velocity to safe limits and tree contact is unavoidable, he should flare the aircraft Ii an extremely nose-high attitude against the densest growth and as close to the ground as possible. In this case, the pilot is using the trees to absorb energy of motion in the horizontal plane and the bottom of the aircraft becomes the main contact point as well as a protective shield. Even individual trees--preferably the smallest ones--can be used for this purpose if the center of the aircraft is aimed at the center of the tree crown. Uprooting a tree under these conditions adds to the impact attenuation process, as shown by accident experience-
As far as less yielding obstacles and man-made obstructions are concerned, the same concepts apply: Avoid nose-first contact under all conditions and avoid destruction of the main rotor until the aircraft is close to the ground and/or the forward velocity is negligible.
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