10. HELICOPTERS. A hovering helicopter generates a downwash from its main rotor (s) similar to the "prop wash" of a conventional aircraft. However, in forward flight, this energy is transformed into a pair of strong, high-speed trailing vortices similar to wing-tip vortices of larger fixed-wing aircraft.
Pilots should avoid helicopter vortices since helicopter forward flight airspeeds are often very low which generate exceptionally strong vortices (figure 18).
FIGURE 18. Helicopter Vortices.
11. JET ENGINE EXHAUST. During ground operations, Jet engine blast (thrust stream turbulence) can cause damage and upsets if encountered at close range.
Exhaust velocity versus distance studies at various thrust levels have shown a need for light aircraft to maintain an adequate separation during ground operations (figure 19).
a. Engine exhaust velocities. generated by larger jet aircrait during ground operations and initial takeoff roll, dictate the desirability of lighter aircraft awaiting takeoff to hold well back of the runway edge at the taxiway hold line. Also, It is desirable to align the aircraft to face any possible jet engine blast effects. Additionally. in the course of running up engines and taxiing on the ground. pilots of larger aircraft should consider the effects of their jet blasts on other aircrait, vehicles, and maintenance and servicing equipment. An illustration of exhaust velocities behind a typical "wide-body" or jumbo jet is shown in figure 19.
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NOTE ALL VELOCITIES ARE IN MILES PER HOUR
SEA LEVEL
STANDARD DAY
ZERO WIND FEET METERS 100740 EDGE OF RUNWA ZERO RAMP GRADIENT 120-3:
100- 30 80 +25 20 -10
120.
140 40 FEET O 100 $00 600 700 800 900 1000 1100 1200 1300 METERS O 90 120 150 180 220 250 280 305 320 350 385 405
- 100 MPM • 70 MPH - SO MPM - 35 мРН
CONVERSION FACTOR I MPM=: 6 XM PER MOUR
JET ENGINE EXHAUST VELOCITY CONTOURS, TAKEOFF POWER FIGURE 19 D. The federal Aviation Administration has established standards tor the location of runway hold lines. For example. runway intersection hold short lines are established 250 feet from the runway centerline for precision approach runways served by approach category C and D aircraft. For runways Are not in distances served by aircrait with wingspans over 171 feet. such as the B-747, taxiway hold lines are 280 feet fror the centerline of precision approach runways.
These hold line distances increases slightly with an increase in field elevation.
12. PILOT RESPONSIBILITY. Government and industry groups are making concerteo efforts to minimize or eliminate the hazards oi tralling vortices.
However. the flight disciplines necessary to ensure vortex avoidance during visual operations must be exercised by the pilot. Vortex visualization and avoidance procedures should be exercised by the pilot using the same degree of concern as in collision avoidance since vortex encounters frequently can 14 Par 1l
a. Pilots are reminded that in operations conducted behind all aircraft, acceptance from Air Traffic Control of traffic'information, instructions to follow an aircraft, or the acceptance of an visual approach clearance, is an acknowledgment that the pilot will ensure safe takeoff and landing intervals and accepts the responsibility of providing his/her own wake turbulence separation.
b. for VER departures behind heavy aircraft, air traffic controllers are required to use at least a 2-minute separation interval unless a pilot has initiated a request to deviate from the 2-minute interval and has indicated acceptance of responsibility for maneuvering his/her aircraft so as to avoid the wake turbulence hazard.
Operational Tips for Light Aircraft How to Avoid Vortex Wake 1 Lift Off Short of Large Aircraft Rotation Point Land Well Beyond Large Aircraft Touchdown Point Pass Over Flight Path of Large Aircraft. or At Least 1000 Under 4 Stay to Windward of Large Aircraft Flight Paths 5 Keep Alert Especially on Calm Days When Vortices Persist Longest AVOID:
1000' FIGURE 20 Davut I Fottin' David S. Potter Acting Director, Flight Standards Service Par 12 15
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