What is an Aiming Angle?

An aiming angle is the angle at which a light fixture points its beam axis, set from a reference line or plane.

An airfield light sends most of its light in one direction. The aiming angle sets that direction. Two numbers define it, one in the horizontal plane and one in the vertical plane.

Horizontal and vertical aiming

• Horizontal aiming. The standards call a horizontal angle toward a line a toe-in. A runway edge light is toed in toward the runway centreline. A light on a taxiway curve is toed in with respect to the tangent of the curve (CS ADR-DSN Figures U-18 and U-20).
• Vertical aiming. The vertical angle is measured up from the horizontal, or from the slope of the surface. For taxiway lights, CS ADR-DSN measures vertical angles from the longitudinal slope of the taxiway surface. It measures horizontal angles from the vertical plane through the taxiway centre line, or from the tangent on a curve.
• Reference. The reference differs by light type. For a PAPI the vertical reference is the glide path. For a runway edge light the horizontal reference is the runway centreline.

Aiming angles by light type

The values below are the ones that the sources state. Many lights have more detail in the manufacturer’s instructions. The FAA tells the installer to follow those instructions for the toe-in of runway edge lights (AC 150/5340-30J, 2.3.1.2.1).

The runway edge light page covers the system. The directional light page covers lights that send their output in one direction.

PAPI aiming

A PAPI differs from the lights above because its vertical aiming angle sets the approach slope that the pilot sees. The FAA gives the aiming angle of each unit in minutes of arc relative to the pre-selected glide path.

A runway with an ILS and height group 4 aircraft uses other L-880 values. They are 35’, 15’, 15’ and 35’. One minute of arc is one sixtieth of a degree. So 30’ is 0.5 degrees. The precision approach path indicator page explains the red and white signal. The PAPI approach slope and elevation setting page explains how the slope is set.

The FAA also says to aim each unit outward into the approach zone. The line is parallel to the runway centerline, within plus or minus 1/2 degree (7.5.4.7.3).

Aiming angle and photometry

Annex 14 and CS ADR-DSN publish an isocandela diagram for each light. The diagram shows the minimum intensity at each angle around the beam axis. The collective notes to the figures say that no deviation is acceptable in the main beam pattern when the lighting fixture is properly aimed.

This links the aim to the measurement. A fixture that points the wrong way puts its main beam in the wrong place. A measurement at one point then shows a different intensity, though the light source is the same.

• The FAA says a misaligned unit looks dimmer or brighter than the units next to it (AC 150/5340-26C, 5.3.2.1.2).
• The FAA says regular photometric measurements are the only practical way to find the misalignment errors of high intensity elevated edge lights.
• EASA asks for in-field measurement of intensity, beam spread and orientation of approach and runway lights. It says to measure all lights as far as practicable.

The photometric testing page covers the instruments and the test rates. The intensity distribution page covers the shape of the diagram.

Tolerances

• Taxiway lights. CS ADR-DSN says to install the light unit with its main beam within one-half degree of the specified requirement. The source is note g of the collective notes to Figures U-16 to U-25.
• PAPI units. The FAA gives plus or minus 1/2 degree in azimuth. EASA says to check the alignment of a visual approach slope indicator system (VASIS) at defined intervals. It says to correct errors above one minute of arc. The same paragraph covers the PAPI and the APAPI (AMC1 ADR.OPS.C.015(a) and (f)).
• Runway centerline lights. The FAA says to align them within 2 degrees of a line parallel to the runway centerline.

Aiming angle, beam angle and aiming point

Three terms use the word angle or aim, and they describe different things.

• Aiming angle gives the direction of the beam axis.
• Beam angle gives the width of the beam. The beam angle page covers it.
• Aiming point marking is a marking on a runway, which has two conspicuous stripes (CS ADR-DSN.L.540). It is not a light setting.

The glide path angle is a fourth term. It is the slope of the approach path. A PAPI aiming angle is set relative to it, but the two are not the same value.

What moves the aim

• Impact. The FAA says mowers and other vehicles can knock elevated lights out of adjustment, and that lenses can move when staff replace lamps.
• Base movement. The aim of an in-pavement light on a shallow base can come out of alignment when the base moves. A light on top of a transformer housing has a non-adjustable aim.
• Soil. The FAA says to check the aim of a PAPI often when the soil freezes or thaws or its moisture changes. Clay soils are the example it gives.

The light fixture page covers the base and the fixture types.

Checking and correcting the aim

If a light looks dimmer or brighter than its neighbours, check its aim. The FAA gives these methods:

1. Look at the lights at night, or measure them with mobile photometric equipment.
2. For in-pavement lights without mobile equipment, turn the lights on in fog. The fog shows the beam, and a mis-aimed light is easy to see.
3. For touchdown zone lights, stand on the centreline and look at the bars on both sides. A light that looks dimmer or brighter than the next one may be mis-aimed.
4. For a PAPI, use the aiming instrument that the maker supplies. Record the angle and the date in a maintenance log.
5. Do photometric testing before and after realignment, to see if the work was needed and if it worked.

EASA also recommends special flight checks after major maintenance of, or changes to, the lighting systems.

TarmacView inspects PAPI and other airfield lights with a drone and reports each light relative to its neighbours. It does not measure intensity in candela. A drone inspection does not replace the photometric test or the aiming instrument. See the drone airport inspection solution.