Illusions in Flight

Reference: AIM 8-1-5

Illusions in flight can related to the vestibular or visual systems. These illusions can lead to spatial disorientation, approach and landing mishaps, and errors in visual scanning and collision avoidance.

Visual Illusions Leading to Landing Errors

Runway Width Illusion: A narrower-than-usual runway can create the illusion that the airplane is too high. The pilot who does not recognize this illusion will fly a lower approach, risking impact with the ground short of the runway. A wider-than-usual runway has the opposite effect.

Runway and Terrain Slopes Illusion: An up-sloping runway can create the illusion that the airplane is higher than it is. The pilot who does not recognize this illusion will fly a lower approach. A down-sloping runway can have the opposite effect.

Featureless Terrain Illusion (“Black Hole Approach”): An absence of ground features can create the illusion that the airplane is at a higher altitude than it is. The pilot who does not recognize these illusions will fly a lower approach.

Atmospheric Illusions:

• Rain on the windshield can create the illusion of greater height, and atmospheric haze can create the illusion of being a greater distance from the runway. The pilot who does not recognize these illusions will fly a lower approach.
• Penetrating fog can create the illusion of pitching up. The pilot who does not recognize this illusion will abruptly steepen the descent.

Ground Lighting Illusions:

• Streetlights or other lights along a straight path can be mistaken for runway or approach lights.
• Bright runway lights may create the illusion of less distance to the runway. The pilot who does not recognize this illusion will fly a higher approach.

Best Practices for Preventing Landing Errors Due to Visual Illusions

• Anticipate visual illusions during approaches to unfamiliar airports, particularly at night or in adverse weather conditions.
• Consult airport diagrams and the Chart Supplement for information on runway slope, terrain, and lighting.
• Make frequent reference to the altimeter and vertical speed indicator.
• Use Visual Approach Slope Indicator (VASI) or Precision Approach Path Indicator (PAPI) systems or an electronic glideslope, whenever they are available.
• Utilize the visual descent point (VDP) found on many nonprecision instrument approach procedure charts. [IFR]

Approach Lighting Systems

References: AIM 2-1-1, 14 CFR 91.175, AC 150/5340-30

Approach lighting systems (ALS) provide pilots with a means to transition from instrument flight to visual flight for landing. Operational requirements dictate the sophistication and configuration of the approach light system for a particular runway.

Definitions

Some approach light systems include sequenced flashing (SF) lights, which appear to the pilot as a ball of light traveling towards the runway at high speed (twice a second). These lights are sometimes called “the rabbit.”

Runway alignment indicator lights (RAIL) are SF lights (typically 5) aligned with the runway centerline. They end where the white approach lights begin (not mixed with steady lights).

Decision bars are horizontal white lights located in some approach light configurations 1,000′ from the threshold. They serve as a visible horizon to ease the transition from instrument flight to visual flight. At this point, the aircraft should be approximately 100′ AGL.

Note: SF lights and RAIL do not continue past the decision bar to avoid distracting the pilot.

Types of Approach Light Systems

Approach Lighting System with Sequenced Flashing Lights (ALSF-1) provides visual information for Category I instrument approaches. It features red terminating bars located on either side of the centerline and 200′ from the end of the runway. Located closer to the runway is another set of red lights called wing bars.

High-Intensity Approach Lighting System with Sequenced Flashing Lights (ALSF-2) provides visual information for Category II and III instrument approaches. The lights extend into the approach area a distance of 2,400′. Strobe lights flash in sequence, starting with the strobe farthest from the runway and ending with the strobe closest to the runway threshold. The lights are spaced at 100-foot intervals from the runway threshold outward to 2,400′.

ALSF-2 features red side row bars. These bars consist of two sets of three red lights on either side of the centerline and extend out 1,000′ from the runway. The bars are in line with the runway touchdown zone (TDZ) lights.

Note: When using the approach lights as the only visual reference, descent below 100′ above TDZE is not authorized until the side row bars (ALSF-2) or the termination bars (ALSF-1) are in view.

Medium Approach Light System with Runway Alignment Indicator Lights (MALSR) is the FAA standard for Category I precision instrument approach runways.

Simplified Short Approach Lighting System with Runway Alignment Indicator Lights (SSALR) is the same configuration as MALSR, but SSALR is combined with a Category II ALSF-2 and is utilized when Category I conditions exist. SSALR is a more economical mode than ALSF-2.

Simplified Short Approach Light System with Sequenced Flashing Lights (SSALF) is the same as SSALR but uses sequenced flashing lights.

Medium Intensity Approach Light System with Sequenced Flashing Lights (MALSF) is a simple, economy-type system equipped with three sequenced flashers at locations where approach area identification problems exist.

Runway Lead-In Light System (RLLS) consists of one or more series of flashing lights installed at or near ground level that provide positive visual guidance along an approach path, either curving or straight, where special problems exist with hazardous terrain, obstructions, or noise abatement procedures. RLLS can be used in conjunction with other lighting systems.

Example: The Canarsie approach to runway 13L at KJFK.

Omni-Directional Approach Lighting System (ODALS) is a configuration of seven omni-directional sequenced flashing lights located in the runway approach area. It provides circling, offset, and straight-in visual guidance for nonprecision approach runways.

Visual Glideslope Indicators

References: 14 CFR 91.129, AIM 2-1-2, AC 150/5340-30

Visual glideslope indicators are located on the left side of some runways to provide the pilot with glidepath information that can be used for day or night approaches.

Note: In Class B, C, and D airspace, pilots who are approaching to land on a runway served by a visual approach slope indicator must maintain an altitude at or above the glidepath until a lower altitude is necessary for a safe landing.

Visual Approach Slope Indicator

The visual approach slope indicator (VASI) is a system of lights arranged to provide a visual descent path to a runway. These lights are visible from about 3–5 miles during the day and up to 20 miles at night.

The VASI’s visual descent path provides safe obstruction clearance within plus or minus 10° of the extended runway centerline and out to 4 NM from the runway threshold. Descent using the VASI should not be initiated until the aircraft is visually aligned with the extended runway centerline.

Two-bar VASI installations are the most common type. They provide one visual glidepath, which is typically set at 3°. At some locations, the angle may be as high as 4.5° to give proper obstacle clearance.

Three-bar VASI installations provide two visual glidepaths. The lower glidepath is provided by the near and middle bars and is typically set at 3°, while the upper glidepath, provided by the middle and far bars, is normally 1/4° higher. This higher glidepath is intended for use only by high-cockpit aircraft to provide a sufficient threshold crossing height.

An easy way to remember VASI indications:

• “Red over white, you’re alright.”
• “White over white, you’ll fly all night.”
• “Red over red, you’re dead.”

Precision Approach Path Indicator

The precision approach path indicator (PAPI) uses light units similar to the VASI but is installed in a single row of either two or four light units. These lights are visible from about 5 miles during the day and up to 20 miles at night.

Safe obstruction clearance is typically provided within plus or minus 10° of the extended runway centerline and to 3.4 NM from the runway threshold. Descent, using the PAPI, should not be initiated until the aircraft is visually aligned with the runway.

The visual glidepath is typically set at 3°, although the angle may be as high as 4.5° at some locations to give proper obstacle clearance.

Visual Descent Points

Reference: AIM 5-4-5

A visual descent point (VDP) is a defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the required visual reference are established.

A VDP is normally identified by DME. Pilots or aircraft not equipped to receive the VDP should fly the approach procedure as though no VDP is provided.

Descent Below the MDA or DA/DH

Reference: 14 CFR 91.175

No pilot may operate an aircraft below the authorized MDA or continue an approach below the authorized DA/DH unless:

• The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under 14 CFR Part 121 or Part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing.
• The flight visibility is not less than the visibility prescribed in the instrument approach being used.
• At least one of the following visual references for the intended runway is distinctly visible and identifiable:
  • The approach light system, except that the pilot may not descend below 100′ above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable;
  • The threshold;
  • The threshold markings;
  • The threshold lights;
  • The runway end identifier lights;
  • The visual approach slope indicator;
  • The touchdown zone or touchdown zone markings;
  • The touchdown zone lights;
  • The runway or runway markings; or
  • The runway lights.

If all of these conditions do not exist, the pilot must immediately execute a missed approach procedure upon arrival at:

• The missed approach point or anytime below the MDA; or
• The DA/DH.

Landing from an Instrument Approach

Reference: 14 CFR 91.175

When to Transition: Pilots may transition to visual references when the runway environment is in sight, the aircraft is in a position to make a normal landing, and the other requirements of 14 CFR 91.175 are met.

Approach Lights: According to 14 CFR 91.175, pilots can descend to 100′ above the touchdown zone elevation (TDZE) if only the approach lights are in sight. To continue below 100′ AGL, the red terminating bars or red side row bars must be visible and identifiable.

Related:

• IFR Arrival and Approach Procedures: Descent Below the MDA or DA/DH
• Airport Signs, Markings, and Lighting: Approach Lighting Systems

Best Practices for Landing from an Instrument Approach

• Use available visual aids such as approach lighting systems, runway lights, and visual approach slope indicators (VASIs) or precision approach path indicators (PAPIs) to ensure proper alignment and descent to the runway.
• During the transition, ensure the aircraft remains on a stable approach path and speed.
• Aim to land within the designated touchdown zone, typically the first third of the runway.

Side-Step Maneuvers

Reference: AIM 5-4-19

ATC may authorize a side-step maneuver to either one of two parallel runways separated by 1,200′ or less, followed by a straight-in landing on the adjacent runway.

Aircraft executing a side-step maneuver are cleared for a specified nonprecision approach with a landing on the adjacent parallel runway. Pilots are expected to commence the side-step maneuver as soon as possible after the runway or runway environment is in sight.

Example Clearance: "[callsign], cleared ILS runway 17 Right approach. Side-step to 17 Left."

Landing minimums to the adjacent runway are based on nonprecision criteria. Therefore, they are higher than the precision minimums for the primary runway but usually are lower than the published circling minimums.

Use of Flaps During Instrument Approaches

Note: Pilots should observe the AFM/POH recommendations and limitations for flap settings during approaches and landings.

Flap Setting: For light GA aircraft, an intermediate flap setting is typically selected just before descending to the MDA or intercepting the glideslope. The partial flap setting is maintained for the landing or until executing a missed approach.

Circling Maneuvers: During circling approaches, flaps may be retracted or held at a position that permits leveling off and maintaining appropriate airspeed. The setting is maintained until the aircraft descends from the circling MDA to make a landing.

Flap Changes: Extending the flaps during an approach must be managed carefully to prevent sudden changes in aircraft attitude and altitude. Knowing specific power settings and trim adjustments helps maintain a smooth, controlled approach.

Testing Standards: The Airmen Certification Standards do not specify flap settings or prohibit the selection of flaps during approaches. The requirement is to maintain a stabilized descent to the MDA or a stabilized visual flight path from the DA/DH to the runway aiming point.

Safety Considerations for Instrument Approaches at Night

• Be aware of potential visual illusions caused by runway lighting, such as the “black hole” effect.
• Use the approach lighting system and PAPI/VASI lights to maintain the proper glide path.
• Familiarize yourself with the different types of runway and approach lighting systems.
• Properly identify runway lights to help maintain alignment and descent accuracy.
• Ensure familiarity with the airport layout, including runway length and orientation.
• Avoid excessive descent rates during the approach, as depth perception can be reduced at night.

Related: Medical Certificates: Visual Illusions Leading to Landing Errors

Common Errors for Landing from an Instrument Approach

• Failure to cross-check the altimeter during the approach and landing phase
• Descending below the DA/DH or MDA without the required visual references
• Misinterpretation of visual cues due to optical illusions
• Failure to maintain a stabilized approach by making excessive pitch or power corrections late in the approach
• Fixation on a single instrument or visual reference, resulting in loss of situational awareness and deviations from the intended flight path
• Failure to cross-check and monitor the instruments during the transition from instrument to visual references for landing
• Incorrect use of or failure to use visual approach slope indicator (VASI) or precision approach path indicator (PAPI) systems when available
• Failing to accomplish all necessary checklist items before beginning the final approach

Risk Examples for Instrument Flight Training

Attempting to Land From an Unstable Approach

• Attempting to land from an unstable approach can result in runway excursions or hard landings; initiate a go-around if the approach is not stabilized.

Flying Below the Glidepath

• Flying below the glidepath increases the risk of CFIT; adhere strictly to glidepath indicators and adjust descent rate to maintain the correct approach path.

Transitioning From Instrument to Visual References for Landing

• Poor timing and abrupt head movements can result in disorientation or runway excursions; shift focus smoothly from the instruments to visual cues.

Aircraft Configuration for Landing

• Abrupt configuration changes can lead to an unstable approach; configure the aircraft as closely as possible for landing before starting the final descent and avoid large, last-minute configuration changes.

Stabilized Approach Concept

A stabilized approach is characterized by a constant-angle, constant-rate of descent approach profile ending near the touchdown point, where the landing maneuver begins. Slight and infrequent adjustments are all that are needed to maintain a stabilized approach.

Stabilized Approach Criteria

C-FLAPS

• Checklists and briefings: Complete
• Flightpath: Established (±1 dot of deflection horizontally and vertically)
• Landing Configuration: Set
• Airspeed: Established (+10/-5 knots)
• Power: Set for the airplane configuration
• Sink Rate: No greater than 1,000 FPM

Notes:

• These parameters should be adjusted for the aircraft type and include all manufacturer guidance.
• Any approach that requires deviations from the parameters should be addressed in a special briefing.

Minimum Stabilization Heights

The recommended minimum stabilization heights are:

• 300′ above the airport elevation in VMC for a small airplane in the traffic pattern.
• 500′ above the airport elevation in VMC.
• 1,000′ above the airfield elevation in IMC.
• For a circling approach, MDA or 500′ above the airport elevation, whichever is lower.

Go-Around for Safety

The objective is to stabilize the aircraft before reaching the predetermined minimum stabilization height. If the aircraft is not stabilized at the minimum stabilization height or becomes unstabilized below it, a go-around should be initiated.

Reference: AC 61-98