of each runway touchdown zone elevation [TDZE]) should be determined to the nearest 0.1 foot from
the MSL. Additionally, runway profiles should be prepared that show elevations listed above the
runway's high and low points, grade changes, and gradients. The elevation of a point on the
instrumented runway centerline nearest to the instrument landing system (ILS) and the glide-path
transmitter are determined to the nearest 0.1 foot from the MSL.
b. Navigational Aids.
Airports requiring airfield obstruction and NAVAID surveys are
instrumented runways. The exact point on the radar, the reflectors, the runway intercepts, or the ILS and
microwave landing system (MLS) components, depends on the type, location, and required accuracy.
The requirement to verify existing systems, their proper description, and all components on or near the
runway is mandatory. With help from airfield operations, maintenance sections, and control-tower
personnel, all information may be obtained for locating and describing all airfield features.
(1) NAVAIDs located on airports include the--
ILS.
MLS.
Precision approach radar (PAR).
Airport surveillance radar (ASR).
(2) NAVAIDs not located on airports include--
Tactical air navigation (TACAN).
Very-high-frequency omnidirectional range (VOR).
Nondirectional beacon (NDB) radios.
Very-high-frequency omnidirectional range and tactical air navigation (VORTAC).
6-2. Obstructions. An obstruction is an object or feature protruding through or above any navigational
imaginary surface that poses a threat to the safe operation of aircraft. Navigational imaginary surfaces
or obstruction identification surfaces (OISs) are defined in FAR-77. Figures 6-1 through 6-3, pages 6-4
through 6-6, show some definitions and samples.
6-3
EN0593
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