---
kind: "section"
citation: "14 C.F.R. § 171.309"
title: "14"
number: "171.309"
heading: "General requirements."
url: "https://uscodex.org/cfr/14/171.309"
---

# §171.309. General requirements.


The MLS is a precision approach and landing guidance system which provides position information and various ground-to-air data. The position information is provided in a wide coverage sector and is determined by an azimuth angle measurement, an elevation angle measurement and a range (distance) measurement.

- (a) An MLS constructed to meet the requirements of this subpart must include:
  - (1) **Approach azimuth equipment, associated monitor, remote control and indicator equipment.**
  - (2) **Approach elevation equipment, associated monitor, remote control and indicator equipment.**
  - (3) A means for the encoding and transmission of essential data words, associated monitor, remote control and indicator equipment. Essential data are basic data words 1, 2, 3, 4, and 6 and auxiliary data words A1, A2 and A3.
  - (4) **Distance measuring equipment (DME), associated monitor, remote control and indicator equipment.**
  - (5) Remote controls for [paragraphs (a)](#a) (1), (2), (3), and (4) of this section must include as a minimum on/off and reset capabilities and may be integrated in the same equipment.
  - (6) At locations where a VHF marker beacon (75 MHz) is already installed, it may be used in lieu of the DME equipment.
- (b) In addition to the equipment required in [paragraph (a)](#a) of this section the MLS may include:
  - (1) **Back azimuth equipment, associated monitor, remote control and indicator equipment.** When Back Azimuth is provided, a means for transmission of Basic Data Word 5 and Auxiliary Data Word A4 shall also be provided.
  - (2) A wider proportional guidance sector which exceeds the minimum specified in §§ [171.313](/cfr/14/171.313.md) and [171.317](/cfr/14/171.317.md).
  - (3) **Precision DME, associated monitor, remote control and indicator equipment.**
  - (4) **VHF marker beacon (75 MHz), associated monitor, remote control and indicator equipment.**
  - (5) The MLS signal format will accommodate additional functions (e.g., flare elevation) which may be included as desired. Remote controls for [paragraphs (b)](#b) (1), (3) and (4) of this section must include as a minimum on/off and reset capabilities, and may be integrated in the same equipment.
  - (6) Provisions for the encoding and transmission of additional auxiliary data words, associated monitor, remote control and indicator equipment.
- (c) MLS ground equipment must be designed to operate on a nominal 120/240 volt, 60 Hz, 3-wire single phase AC power source and must meet the following service conditions:
  - (1) **AC line parameters, DC voltage, elevation and duty—**
  - (2) **Ambient conditions within the shelter for electronic equipment installed in shelters are—**
  - (3) Ambient conditions for electronic equipment and all other equipment installed outdoors (for example, antenna, field detectors, and shelters):
  - (4) All equipment installed outdoors must operate satisfactorily under the following conditions:
- (d) The transmitter frequencies of an MLS must be in accordance with the frequency plan approved by the FAA.
- (e) The DME component listed in [paragraph (a)(4)](#a-4) of this section must comply with the minimum standard performance requirements specified in [subpart G](/cfr/14/subpartG.md) of this part.
- (f) The marker beacon components listed in [paragraph (b)(4)](#b-4) of this section must comply with the minimum standard performance requirements specified in [subpart H](/cfr/14/subpartH.md) of this part.

## Notes

### Source

Source: Docket 20669, 51 FR 33177, Sept. 18, 1986, unless otherwise noted.

### Authority

Authority: 49 U.S.C. 106(g), 40103-40107, 40109, 40113, 44502, 44701-44702, 44708-44709, 44711, 44719-44721, 45303, 46308.

### Source

Source: Docket 5034, 29 FR 11337, Aug. 6, 1964, unless otherwise noted.