c

OCKPIT CLASSROOM

999

By Harold Holmes

SAFE TAXIING Safe taxiing appears to be a fairly simple maneuver compared with most other aircraft maneuvers. However, accident statistics bear out the fact that taxi accidents are quite numerous. Collisions with other taxiing or parked aircraft happen all too often. Also, improper taxi procedures such as riding the brakes or using improper braking while turning is the cause of costly maintenance repairs to the mechanical linkage of the nose gear assembly, uneven wear of tires and brake repairs. The purpose of this article is to point out some safe taxiing procedures which will reduce accidents and maintenance expenses. It requires much more power to start the aircraft moving or to start a turn than it does to keep it moving in any given direction. For this reason, all controls, including the throttle, rudder pedals and brakes, must be used properly. Check your brakes before moving more than the length of your aircraft. Particular care must be taken in the use of the throttle while taxiing to prevent overheating and possible mechanical damage to the engine.

TAXIING Objective: Taxiing is the controlled movement of the airplane under its own power, while on the ground.

(Position of control while taxiing) •Black arrow indicates direction of wind. Light airplanes are provided with nose-wheel steering capability. This is done through a simple system of mechanical linkage connected to the rudder pedals. Push-pull rods connect the pedals to fittings located on the pivotal portion of the nosewheel strut. 34 AUGUST 1985

Taxiing with the steerable nosewheel now common on tricycle-gear airplanes requires little special piloting technique, but the habitual observance of all safety precautions is necessary, just as it is with tailwheel type airplanes. It is generally poor technique to use excessively high power settings and attempt to control the airplane's direction and taxi speed with the brakes. The flight controls are effective to a greater or lesser degree, depending upon the taxi speed and velocity and relative direction of the wind. Caution is required when taxiing light tricycle type gear airplanes, especially the high wing type, in strong quartering tailwinds. Such a wind can cause an airplane to flip over on its back unless the ailerons, rudder and elevators are positioned properly. To prevent the possibility of a flipover when taxiing with a strong quartering tailwind, the elevator should be in a down position, with the elevator control forward. In addition, the aileron on the side from which the wind is blowing should be in the down position, aileron control moved in the direction opposite than that from which the wind is blowing. Also, sudden bursts of the throttle and sharp braking should be avoided when taxiing with strong quartering tailwinds. Except while taxiing very slowly, it is best to slow down before attempting a

turn, otherwise the turn is likely to be too sharp or too prolonged, particularly when turning from a downwind heading to an upwind heading. It is best to slow down before attempting to turn because of the tendency to weathervane. Excessive braking while turning can cause nosewheel problems and unnecessary tire wear. In turning from an upwind heading to a downwind heading, however, this precaution for slowing down is not as important, since the same tendency will cause a deceleration of the turn. One of the major considerations in the taxiing of airplanes with the tailwheel gear is its tendency to weathervane while it is being taxied crosswind. This effect may be much less or even negligible when taxiing airplanes with a tricycle landing gear. While the airplane is moving on the ground it is considerably affected by the direction and velocity of the wind. When taxiing into the wind the control effectiveness is increased by the speed of the wind as well as the tendency of all planes to weathervane. The effect of weathervaning makes the effective

OFFICIAL DESIGNATIONS OF STRENGTH OF WINDS Strength of winds are designated in three ways: 1) by .Beaufort Scale Number; 2) by National Weather Service designations; and 3) by velocity in miles per hour

BEAUFORT SCALE

NATIONAL WEATHER SERVICE

VELOCITY IN MILES

DESIGNATIONS

PER HOW

Oto 1 Ito3

0

CALM . . . . . . . . . . . . . . .

1

LIGHT AIR . . . . . . . . . . . . .

2

LIGHT BREEZE . . . . . . . . . . .

3

GENTLE BREEZE . . . . . . . . .

4

MODERATE BREEZE. . . . . . .

5

FRESH BREEZE. . . . . . . . . .

4 to 8 to 13 to 19 to

6

STRONG BREEZE . . . . . . . .

25 to 31

7

MODERATE GALE. . . . . . . . .

8

FRESH GALE . . . . . . . . . . .

7 12 18 24

9

STRONG GALE . . . . . . . . . .

10

WHOLE GALE . . . . . . . . . . .

32 to 39 to 47 to 55 to

38 46 54 63

11

STORM . . . . . . . . . . . . . . .

64 to 73

12

HURRICANE . . . . . . . . . . . .

JET BLAST/JET INTAKE HAZARDS INGESTION DANGER AREA

CROSSWIND

25' SAFETY RADIUS CURRENT IET AIRCRAFT AT ALL THRUST SETTINGS

IDLE BUST DANCER AREA

s s

1

i (IDLE)

2M' B-747 ISO' OTHER AIRCRAFT

(THRUST)

3 5 O

375' B-747

(TAKE OFF)

2N' OTHER AIRCRAFT

(THRUST)

5 T •O

s

o

I

°

»r—

z

• Correct airplane's tendency to weathervane with rudder and brakes. (Brakes important.) • Check Flight Manual for maximum crosswind conditions.

wind velocity seem greater, and makes taxiing into the wind easier than taxiing in other directions. The tendency for a plane to weathervane is greatest while taxiing directly crosswind, which makes this maneuver extremely difficult without brakes. It is almost impossible to keep the plane from turning into even a moderate breeze (13 to 18 mph). In taxiing downwind, the tendency to weathervane seems to be increased as the velocity of the tailwind lessens the effectiveness of the controls. When the wind velocity is greater than the groundspeed, control surfaces are exposed to what is called a "reversed wind" condition, meaning that when an aileron is in an up position a tailwind will tend to lift that wing; whereas a wind from the front of the aircraft with aileron "up" would force the wing down. When taxiing downwind with the speed of the wind greater than taxiing speed, use forward stick with elevators down. In the event of a direct tailwind the ailerons should be in a neutral position. It is difficult to set any rule for a safe taxiing speed which is usually no faster than a person walking rapidly. What is safe under some conditions may be hazardous under others (always taxi

stop or turn where and when desired. Movement of the plane should be dependent on the throttle. That is, the speed should be slow enough to enable you to stop the airplane promptly when the throttle is closed or the aircraft will stop on its own when the throttle is closed. One other thing to watch for in regard to taxiing speed is that an airplane should taxi no faster than would be safe without brakes — just in case the brakes do fail. Usually this means, as mentioned earlier, at a walking pace but it depends a lot on the circumstances . . . how many other aircraft are on or near the taxiways, the length and width of the taxiway, etc. If the clearance between objects appears too narrow, it probably is, so STOP — take a look and use a guide to assist you. Sharp turns and attempts to turn at too great a speed should be avoided since both tend to exert excessive strains on the airplane and are difficult to control once started. In many airplanes with tricycle landing gears, the arc around which the nosewheel will pivot is restricted — damage done to the mechanical linkage of the nose gear is expensive. This may make it impossible to pivot the airplane

safe taxiing is safe control; the ability to

any event, and will require a larger area

cautiously). The primary requisite of

on one wheel, which is poor practice in

for maneuvering. Generally, brakes and power should not be used at the same time — locking a brake and pivoting around one wheel is not recommended. In starting to taxi a tricycle gear airplane, the airplane should be allowed to roll forward slowly to enable the nosewheel to center and to prevent the possibility of a swerve into another airplane or nearby obstruction. Pilots should realize that airplanes are not at home on the ground. Taxiing properly requires a fine blending of speed control and throttle use, as well as a well developed sense of anticipation of the tendencies of the aircraft. Remember, it requires much more power to start the aircraft into motion (1100-1300 rpm) or to start a turn than it does to keep it moving (800-1000 rpm) in any given direction. However, care must be taken to avoid excessive

use of the throttle and thus overheating

and causing mechanical damage to the engine. Even/time the throttle is closed when going either downwind or crosswind, the forces causing the weathervaning tendencies immediately act to reduce effective control and cause involuntary

turning. For the rudder to be effective while taxiing it must have airflow pass-

ing over it. In some cases a blast of air from the prop will aid in starting a turn SPORT AVIATION 35

or making a correction while taxiing. Jet blast on an airport ramp is a definite hazard to a taxiing light aircraft. Jet blast can have a velocity of 590 mph and a temperature of 370 degrees at a point 25 feet behind the engine. At a distance of 100 feet, blast velocity is 95 mph and the temperature about 140 degrees F. For safety reasons, light planes should not taxi closer than 1000 feet (length of 3 football fields) behind a taxiing jet transport aircraft. Note the differences in the "idle blast" and 'lake off" blast danger areas. Remember that prop blast from a heavier airplane can also present a hazard to light airplanes. In previous "Cockpit Classroom" articles we have discussed flight in windy or gusty conditions. Taxiing during windy or gusty conditions should cause pilots to use extreme care in deciding whether or not the winds are too strong for taxiing. A conservative rule which can be used to cover most light airplanes is to compute 40% x the stall speed of the plane. Let's say, for example, the stall speed is 50 knots — 40% x 50 = 20 knots. Taxiing should not be attempted with winds over 23 mph. Strong crosswinds, above 20 mph, will present a problem of directional control requiring downwind brake to prevent the airplane from turning into the wind. If this happens, the brake should be periodically released allowing the brake to cool. Operating on the ground during higher than normal or gusty wind conditions or in close proximity to large and turbine-powered aircraft can be particularly hazardous for light airplanes. The use of safe taxiing procedures on the ground is the sign of a good pilot. These procedures include the proper use of throttle and brakes, allowing enough room for turns without excessive braking, positioning of the flight controls properly during windy conditions, handling the weathervaning tendency and preventing a ground loop when landing. Braking action during the landing roll should be considered to be a secondary function — use brakes only if necessary.

USE OF THROTTLE — Hold throttle control so that small changes (a few millimeters) can be made smoothly.

Use several fingers to act as a stop in contact with panel or quadrant. (Note: Do not hold throttle entirely by its knob or lever handle) Check with your instructor for proper demonstration.

USE OF RUDDER — Rudder pedals are the primary directional controls while taxiing. Rest heels on the floor and balls of feet on bottom of rudder pedals. Use firm movements with feet on pedals.

USE OF BRAKES — Move foot to top of rudder and depress or use handbrake when it is necessary to stop.

Check braking action shortly after starting to taxi. Emergency stop — both brakes should be applied simultaneously. Apply brakes smoothly, evenly and cautiously.

BRAKES USED FOR: • Slowing • Stopping

• Holding • Steering

Remember that a surprising number of accidents or incidents occur annually during taxi operations. Is it because pilots feel that taxiing is such an elementary operation that they have become complacent and inattentive to safe taxi procedures and hazards on the ground? Probably . . . all of us who fly need to concentrate more on keeping alert while taxiing.

DOWNWIND

Taxi with yoke neutral or slightly forward (watch weather-vaning tendency because tail wind lessens effectiveness of controls). The stronger the tail wind the more the elevators are depressed.

> In strong headwinds move control wheel slightly aft of neutral to control pitch attitude while taxiing.

UPWIND

In strong headwinds move the yoke slightly aft of neutral.

Note—Do not taxi at speeds in excess of 15-20 knots in wind of this same velocity.

36 AUGUST 1985

CHECK BRAKES FOR:

1. Sufficient force to stop aiprlane. 2. Hold airplane during run-ups. 3. Permit steering while taxiing.

Evaluation • Check brakes prior to each flight by braking plane to near stop during taxi. • Use low power settings, 1000 rpm. • When brakes are used, close throttle. • Use brakes to assist in turning and stopping (after landing — use brakes when speed is under taxiing control). • Keep speed under control at all

times (fast walking speed). No faster than would be safe without brakes. • Slow down — before attempting turn — no sharp turns. • Taxi slowly when attendant is assisting at the wing in strong winds. • Avoid sudden burst of throttle and sharp braking especially in strong quartering tailwinds. • Observe local taxi rules and tower instructions. • Adhere to taxi signals and clearances. • Recognize and avoid hazards. • Avoid creating hazards to persons or property.