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AIM

10/12/17

7−5−5

Potential Flight Hazards

1. Mountain waves occur when air is being

blown over a mountain range or even the ridge of a

sharp bluff area. As the air hits the upwind side of the

range, it starts to climb, thus creating what is

generally a smooth updraft which turns into a

turbulent downdraft as the air passes the crest of the

ridge. From this point, for many miles downwind,

there will be a series of downdrafts and updrafts.

Satellite photos of the Rockies have shown mountain

waves extending as far as 700 miles downwind of the

range. Along the east coast area, such photos of the

Appalachian chain have picked up the mountain

wave phenomenon over a hundred miles eastward.

All it takes to form a mountain wave is wind blowing

across the range at 15 knots or better at an intersection

angle of not less than 30 degrees.

2. Pilots from flatland areas should understand

a few things about mountain waves in order to stay

out of trouble. When approaching a mountain range

from the upwind side (generally the west), there will

usually be a smooth updraft; therefore, it is not quite

as dangerous an area as the lee of the range. From the

leeward side, it is always a good idea to add an extra

thousand feet or so of altitude because downdrafts

can exceed the climb capability of the aircraft. Never

expect an updraft when approaching a mountain

chain from the leeward. Always be prepared to cope

with a downdraft and turbulence.

3. When approaching a mountain ridge from the

downwind side, it is recommended that the ridge be

approached at approximately a 45 degree angle to the

horizontal direction of the ridge. This permits a safer

retreat from the ridge with less stress on the aircraft

should severe turbulence and downdraft be experi-

enced. If severe turbulence is encountered,

simultaneously reduce power and adjust pitch until

aircraft approaches maneuvering speed, then adjust

power and trim to maintain maneuvering speed and

fly away from the turbulent area.

7−5−7. Use of Runway Half−way Signs at

Unimproved Airports

When installed, runway half−way signs provide the

pilot with a reference point to judge takeoff

acceleration trends. Assuming that the runway length

is appropriate for takeoff (considering runway

condition and slope, elevation, aircraft weight, wind,

and temperature), typical takeoff acceleration should

allow the airplane to reach 70 percent of lift−off

airspeed by the midpoint of the runway. The “rule of

thumb” is that should airplane acceleration not allow

the airspeed to reach this value by the midpoint, the

takeoff should be aborted, as it may not be possible to

liftoff in the remaining runway.

Several points are important when considering using

this “rule of thumb”:

a. Airspeed indicators in small airplanes are not

required to be evaluated at speeds below stalling, and

may not be usable at 70 percent of liftoff airspeed.

b. This “rule of thumb” is based on a uniform

surface condition. Puddles, soft spots, areas of tall

and/or wet grass, loose gravel, etc., may impede

acceleration or even cause deceleration. Even if the

airplane achieves 70 percent of liftoff airspeed by the

midpoint, the condition of the remainder of the runway

may not allow further acceleration. The entire length

of the runway should be inspected prior to takeoff to

ensure a usable surface.

c. This “rule of thumb” applies only to runway

required for actual liftoff. In the event that obstacles

affect the takeoff climb path, appropriate distance

must be available after liftoff to accelerate to best angle

of climb speed and to clear the obstacles. This will, in

effect, require the airplane to accelerate to a higher

speed by midpoint, particularly if the obstacles are

close to the end of the runway. In addition, this

technique does not take into account the effects of

upslope or tailwinds on takeoff performance. These

factors will also require greater acceleration than

normal and, under some circumstances, prevent

takeoff entirely.

d. Use of this “rule of thumb” does not alleviate the

pilot’s responsibility to comply with applicable

Federal Aviation Regulations, the limitations and

performance data provided in the FAA approved

Airplane Flight Manual (AFM), or, in the absence of

an FAA approved AFM, other data provided by the

aircraft manufacturer.

In addition to their use during takeoff, runway

half−way signs offer the pilot increased awareness of

his or her position along the runway during landing

operations.