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operating parameters. It means an increase in runway
length requirements for takeoff and landings, and a
decreased rate of climb. An average small airplane,
for example, requiring 1,000 feet for takeoff at sea
level under standard atmospheric conditions will
require a takeoff run of approximately 2,000 at an
operational altitude of 5,000 feet.
NOTE−
A turbo−charged aircraft engine provides some slight
advantage in that it provides sea level horsepower up to a
specified altitude above sea level.
6.6 Density Altitude Advisories. At airports with
elevations of 2,000 feet and higher, control towers
and FSSs will broadcast the advisory “Check Density
Altitude” when the temperature reaches a predetermined
level. These advisories will be broadcast on
appropriate tower frequencies or, where available,
ATIS. FSSs will broadcast these advisories as a part
of Airport Advisory Service, and on TWEB.
6.6.1 These advisories are provided by air traffic
facilities, as a reminder to pilots that high
temperatures and high field elevations will cause
significant changes in aircraft characteristics. The
pilot retains the responsibility to compute density
altitude, when appropriate, as a part of preflight
duties.
NOTE−
All FSSs will compute the current density altitude upon
request.
7. Use of Runway Half−way Signs at
Unimproved Airports
7.1 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
31 JULY 08
AIP ENR 5.7−5
United States of America 15 MAR 07
Federal Aviation Administration Nineteenth Edition
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.
7.2 Several points are important when considering
using this “rule of thumb”:
7.2.1 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.
7.2.2 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.
7.2.3 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.
7.2.4 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.
7.3 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.
NOTE−
No FAA standard exists for the appearance of the runway
half−way sign. FIG ENR 5.7−1 shows a graphical
depiction of a typical runway half−way sign.
FIG ENR 5.7−1
Typical Runway Half−way Sign
8. Mountain Wave
8.1 Many pilots go all their lives without understanding
what a mountain wave is. Quite a few have lost
their lives because of this lack of understanding. One
need not be a licensed meteorologist to understand
　
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