Pilot's Handbook of Aeronautical Knowledge (Federal Aviation Administration)

by Federal Aviation Administration

  Figure 14-44. Traffic advisories.

  Wake Turbulence

  All aircraft generate wake turbulence during flight. This disturbance is caused by a pair of counter-rotating vortices trailing from the wingtips. The vortices from larger aircraft pose problems to encountering aircraft. The wake of these aircraft can impose rolling moments exceeding the roll-control authority of the encountering aircraft. Also, the turbulence generated within the vortices can damage aircraft components and equipment if encountered at close range. For this reason, a pilot must envision the location of the vortex wake and adjust the flight path accordingly.

  Vortex Generation

  Lift is generated by the creation of a pressure differential over the wing surface. The lowest pressure occurs over the upper wing surface and the highest pressure under the wing. This pressure differential triggers the rollup of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips. After the rollup is completed, the wake consists of two counter rotating cylindrical vortices. Most of the energy lies within a few feet of the center of each vortex. [Figure 14-45]

  Figure 14-45. Vortex generation.

  Vortex Strength

  Terminal Area

  Wake turbulence has historically been thought of as only a function of aircraft weight, but recent research considers additional parameters, such as speed, aspects of the wing, wake decay rates, and aircraft resistance to wake, just to name a few. The vortex characteristics of any aircraft will be changed with the extension of flaps or other wing configuration devices, as well as changing speed. However, as the basic factors are weight and speed, the vortex strength increases proportionately with an increase in aircraft operating weight or decrease in aircraft speed. The greatest vortex strength occurs when the generating aircraft is heavy, slow, and clean, since the turbulence from a “dirty” aircraft configuration hastens wake decay.

  En Route

  En route wake turbulence events have been influenced by changes to the aircraft fleet mix that have more “Super” (A380) and “Heavy” (B-747, B-777, A340, etc.) aircraft operating in the NAS. There have been wake turbulence events in excess of 30NM and 2000 feet lower than the wake generating aircraft. Air density is also a factor in wake strength. Even though the speeds are higher in cruise at high altitude, the reduced air density may result in wake strength comparable to that in the terminal area. In addition, for a given separation distance, the higher speeds in cruise result in less time for the wake to decay before being encountered by a trailing aircraft.

  Vortex Behavior

  Trailing vortices have certain behavioral characteristics that can help a pilot visualize the wake location and take avoidance precautions.

  Vortices are generated from the moment an aircraft leaves the ground (until it touches down), since trailing vortices are the byproduct of wing lift. [Figure 14-46] The vortex circulation is outward, upward, and around the wingtips when viewed from either ahead or behind the aircraft. Tests with large aircraft have shown that vortices remain spaced a bit less than a wingspan apart, drifting with the wind, at altitudes greater than a wingspan from the ground. Tests have also shown that the vortices sink at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft.

  Figure 14-46. Vortex behavior.

  When the vortices of larger aircraft sink close to the ground (within 100 to 200 feet), they tend to move laterally over the ground at a speed of 2–3 knots. A crosswind decreases the lateral movement of the upwind vortex and increases the movement of the downwind vortex. A light quartering tailwind presents the worst case scenario as the wake vortices could be all present along a significant portion of the final approach and extended centerline and not just in the touchdown zone as typically expected.

  Vortex Avoidance Procedures

  The following procedures are in place to assist pilots in vortex avoidance in the given scenario.

  • Landing behind a larger aircraft on the same runway—stay at or above the larger aircraft’s approach flight path and land beyond its touchdown point. [Figure 14-47A]

  • Landing behind a larger aircraft on a parallel runway closer than 2,500 feet—consider the possibility of drift and stay at or above the larger aircraft’s final approach flight path and note its touchdown point. [Figure 14-47B]

  • Landing behind a larger aircraft on crossing runway—cross above the larger aircraft’s flight path.

  • Landing behind a departing aircraft on the same runway—land prior to the departing aircraft’s rotating point.

  • Landing behind a larger aircraft on a crossing runway—note the aircraft’s rotation point and, if that point is past the intersection, continue and land prior to the intersection. If the larger aircraft rotates prior to the intersection, avoid flight below its flight path. Abandon the approach unless a landing is ensured well before reaching the intersection. [Figure 14-47C]

  • Departing behind a large aircraft—rotate prior to the large aircraft’s rotation point and climb above its climb path until turning clear of the wake.

  • For intersection takeoffs on the same runway—be alert to adjacent larger aircraft operations, particularly upwind of the runway of intended use. If an intersection takeoff clearance is received, avoid headings that cross below the larger aircraft’s path.

  • If departing or landing after a large aircraft executing a low approach, missed approach, or touch-and-go landing (since vortices settle and move laterally near the ground, the vortex hazard may exist along the runway and in the flight path, particularly in a quartering tailwind), it is prudent to wait at least 2 minutes prior to a takeoff or landing.

  • En route, it is advisable to avoid a path below and behind a large aircraft, and if a large aircraft is observed above on the same track, change the aircraft position laterally and preferably upwind.

  Collision Avoidance

  Title 14 of the CFR part 91 has established right-of-way rules, minimum safe altitudes, and VFR cruising altitudes to enhance flight safety. The pilot can contribute to collision avoidance by being alert and scanning for other aircraft. This is particularly important in the vicinity of an airport.

  Effective scanning is accomplished with a series of short, regularly spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should not exceed 10°, and each should be observed for at least 1 second to enable detection. Although back and forth eye movements seem preferred by most pilots, each pilot should develop a scanning pattern that is most comfortable and then adhere to it to assure optimum scanning. Even if entitled to the right-of-way, a pilot should yield if another aircraft seems too close.

  Clearing Procedures

  The following procedures and considerations are in place to assist pilots in collision avoidance under various situations:

  • Before takeoff—prior to taxiing onto a runway or landing area in preparation for takeoff, pilots should scan the approach area for possible landing traffic, executing appropriate maneuvers to provide a clear view of the approach areas.

  • Climbs and descents—during climbs and descents in flight conditions that permit visual detection of other traffic, pilots should execute gentle banks left and right at a frequency that permits continuous visual scanning of the airspace.

  • Straight and level—during sustained periods of straight-and-level flight, a pilot should execute appropriate clearing procedures at periodic intervals.

  • Traffic patterns—entries into traffic patterns while descending should be avoided.

  • Traffic at VOR sites—due to converging traffic, sustained vigilance should be maintained in the vicinity of VORs and intersections.

  • Training operations—vigilance should be maintained and clearing turns should be made prior to a practice maneuver. During instruction, the pilot should be asked to verbalize the clearing procedures (call out “clear left, right, above, and below�
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  Figure 14-47. Vortex avoidance procedures.

  High-wing and low-wing aircraft have their respective blind spots. The pilot of a high-wing aircraft should momentarily raise the wing in the direction of the intended turn and look for traffic prior to commencing the turn. The pilot of a low-wing aircraft should momentarily lower the wing and look for traffic prior to commencing the turn.

  Pilot Deviations (PDs)

  A pilot deviation (PD) is an action of a pilot that violates any Federal Aviation Regulation. While PDs should be avoided, the regulations do authorize deviations from a clearance in response to a traffic alert and collision avoidance system resolution advisory. You must notify ATC as soon as possible following a deviation.

  Pilot deviations can occur in several different ways. Airborne deviations result when a pilot strays from an assigned heading or altitude or from an instrument procedure, or if the pilot penetrates controlled or restricted airspace without ATC clearance.

  To prevent airborne deviations, follow these steps:

  • Plan each flight—you may have flown the flight many times before but conditions and situations can change rapidly, such as in the case of a pop-up temporary flight restriction (TFR). Take a few minutes prior to each flight to plan accordingly.

  • Talk and squawk—Proper communication with ATC has its benefits. Flight following often makes the controller’s job easier because they can better integrate VFR and IFR traffic.

  • Give yourself some room—GPS is usually more precise than ATC radar. Using your GPS to fly up to and along the line of the airspace you are trying to avoid could result in a pilot deviation because ATC radar may show you within the restricted airspace.

  Ground deviations (also called surface deviations) include taxiing, taking off, or landing without clearance, deviating from an assigned taxi route, or failing to hold short of an assigned clearance limit. To prevent ground deviations, stay alert during ground operations. Pilot deviations can and frequently do occur on the ground. Many strategies and tactics pilots use to avoid airborne deviations also work on the ground.

  Pilots should also remain vigilant about vehicle/pedestrian deviations (V/PDs). A vehicle or pedestrian deviation includes pedestrians, vehicles or other objects interfering with aircraft operations by entering or moving on the runway movement area without authorization from air traffic control. In serious instances, any ground deviation (PD or VPD) can result in a runway incursion. Best practices in preventing ground deviations can be found in the following section under runway incursion avoidance.

  Runway Incursion Avoidance

  A runway incursion is “any occurrence in the airport runway environment involving an aircraft, vehicle, person, or object on the ground that creates a collision hazard or results in a loss of required separation with an aircraft taking off, intending to take off, landing, or intending to land.” It is important to give the same attention to operating on the surface as in other phases of flights. Proper planning can prevent runway incursions and the possibility of a ground collision. A pilot should always be aware of the aircraft’s position on the surface at all times and be aware of other aircraft and vehicle operations on the airport. At times, towered airports can be busy and taxi instructions complex. In this situation, it may be advisable to write down taxi instructions. The following are some practices to help prevent a runway incursion:

  • Read back all runway crossing and/or hold instructions.

  • Review airport layouts as part of preflight planning, before descending to land and while taxiing, as needed.

  • Know airport signage.

  • Review NOTAM for information on runway/taxiway closures and construction areas.

  • Request progressive taxi instructions from ATC when unsure of the taxi route.

  • Check for traffic before crossing any runway hold line and before entering a taxiway.

  • Turn on aircraft lights and the rotating beacon or strobe lights while taxing.

  • When landing, clear the active runway as soon as possible, then wait for taxi instructions before further movement.

  • Study and use proper phraseology in order to understand and respond to ground control instructions.

  • Write down complex taxi instructions at unfamiliar airports.

  Approximately three runway incursions occur each day at towered airports within the United States. The potential that these numbers present for a catastrophic accident is unacceptable. The following are examples of pilot deviations, operational incidents (OI), and vehicle (driver) deviations that may lead to runway incursions.

  Pilot Deviations:

  • Crossing a runway hold marking without clearance from ATC

  • Taking off without clearance

  • Landing without clearance

  Operational Incidents (OI):

  • Clearing an aircraft onto a runway while another aircraft is landing on the same runway

  • Issuing a takeoff clearance while the runway is occupied by another aircraft or vehicle

  Vehicle (Driver) Deviations:

  • Crossing a runway hold marking without ATC clearance

  According to FAA data, approximately 65 percent of all runway incursions are caused by pilots. Of the pilot runway incursions, FAA data shows almost half of those incursions are caused by GA pilots.

  Causal Factors of Runway Incursions

  Detailed investigations of runway incursions over the past 10 years have identified three major areas contributing to these events:

  • Failure to comply with ATC instructions

  • Lack of airport familiarity

  • Nonconformance with standard operating procedures

  Clear, concise, and effective pilot/controller communication is paramount to safe airport surface operations. You must fully understand and comply with all ATC instructions. It is mandatory to read back all runway “hold short” instructions verbatim.

  Taxiing on an unfamiliar airport can be very challenging, especially during hours of darkness or low visibility. A request may be made for progressive taxi instructions which include step by step taxi routing instructions. Ensure you have a current airport diagram, remain “heads-up” with eyes outside, and devote your entire attention to surface navigation per ATC clearance. All checklists should be completed while the aircraft is stopped. There is no place for non-essential chatter or other activities while maintaining vigilance during taxi. [Figure 14-48]

  Runway Confusion

  Runway confusion is a subset of runway incursions and often results in you unintentionally taking off or landing on a taxiway or wrong runway. Generally, you are unaware of the mistake until after it has occurred.

  Figure 14-48. Heads-up, eyes outside.

  In August 2006, the flight crew of a commercial regional jet was cleared for takeoff on Runway 22 but mistakenly lined up and departed on Runway 26, a much shorter runway. As a result, the aircraft crashed off the end of the runway.

  Causal Factors of Runway Confusion

  There are three major factors that increase the risk of runway confusion and can lead to a wrong runway departure:

  • Airport complexity

  • Close proximity of runway thresholds

  • Joint use of a runway as a taxiway

  Not only can airport complexity contribute to a runway incursion; it can also play a significant role in runway confusion. If you are operating at an unfamiliar airport and need assistance in executing the taxi clearance, do not hesitate to ask ATC for help. Always carry a current airport diagram and trace or highlight your taxi route to the departure runway prior to leaving the ramp.

  If you are operating from an airport with runway thresholds in close proximity to one another, exercise extreme caution when taxiing onto the runway. Figure 14-49 shows a perfect example of a taxiway leading to multiple runways that may cause confusion. If departing on Runway 36, ensure that you set your aircraft heading “bug” to 360°, and align your aircraft to the runway heading to avoid
departing from the wrong runway. Before adding power, make one last instrument scan to ensure the aircraft heading and runway heading are aligned. Under certain circumstances, it may be necessary to use a runway as a taxiway. For example, during airport construction some taxiways may be closed requiring rerouting of traffic onto runways. In other cases, departing traffic may be required to back taxi on the runway in order to utilize the full runway length.

  Figure 14-49. Confusing runway/runway intersection.

  Since inattention and confusion often are factors contributing to runway incursion, it is important to remain extremely cautious and maintain situational awareness (SA). When instructed to use a runway as a taxiway, do not become confused and take off on the runway you are using as a taxiway.

  ATC Instructions

  Title 14 of the Code of Federal Regulations (14 CFR) part 91, section 91.123 requires you to follow all ATC clearances and instructions. Request clarification if you are unsure of the clearance or instruction to be followed. If you are unfamiliar with the airport or unsure of a taxi route, ask ATC for a “progressive taxi.” Progressive taxi requires the controller to provide step-by-step taxi instructions.