ILS & DME systems


The student should gain an understanding of DME operation and the instrument landing system, and be able to apply that knowledge to flight scenarios.


  • Radio theory review
  • CDI display review
  • ILS

    • Localizer
    • Glideslope
    • Marker beacons
    • Runway markings
    • Approach lighting
  • DME

    • General usage
    • Arcing


Instructor actions

  • Introduce and motivate with the question of how we gauge distance and arrive at the airport without visual references
  • Describe new navigation systems and their operation in a predominantly lecture format
  • Illustrate techniques using Tim's simulator
  • Review each system before moving on to the next
  • Evaluate student knowledge with questions emphasizing understanding rather than rote

    • Have the student solve multiple scenarios
  • Conclude with an oral quiz, identifying and correcting errors

Student actions

  • Arrive with completed homework assignment
  • Maintain active involvement by responding to questions and taking notes
  • Guide the instructor through the intercepting and tracking of several simulator scenarios
  • Complete an oral quiz and demonstration of the concepts

Completion Standards

The lesson will be complete when the student can describe DME and ILS equipment - and demonstrate an understanding of the techniques required to smoothly and accurately intercept and track a localizer and glideslope - with minimal instructor guidance.

Teaching outline

Review radio principles

  • What are the three types of waves?
  • Which kind of frequency does the ILS use? How about DME?

CDI with localizer and glideslope

  • Not all CDIs can be used with an ILS, but all should work with a localizer

    • when flying a localizer, it is four times as sensitive - every dot is .5° rather than 2°
    • the glideslope needle will be flown just like a normal lateral course - the center is the airplane
    • performance instrument, used as a reference to adjust control inputs

Instrument Landing System, IF 309 & IFH 7-27

The ILS is a precision approach, providing both lateral and vertical guidance down a predetermined flight path.

  • As a precision approach, it is designed to transition from instrument to visual flight
  • Uses a system of 'lobes'; overlapping points define the approach

    • VHF band
    • operates between 108.10 and 111.9 MHz

  • Five parts make up the ILS in actual use:

    1. localizer provides lateral (directional) guidance

      • broadcast from the non-arrival end of the runway (illustration from IFH 7-28)
      • 40 channels, on odd tenths between 108.10 and 111.95
      • the 'full scale' of 5° will extend to 700 ft wide at the threshold, varying the localizer width
      • service volume reaches 18 nm, between 1000 agl and 4500 above the antenna
      • lateral area is ±10° to 18nm, ±35° to 10 nm, providing proper off-course indications within the volume
    2. glideslope provides vertical guidance with a UHF signal

      • broadcast from antennas located approximately 1,000 ft from the approach end
      • 40 channels, paired with the localizer
      • full deflection occurs .7° high or low; useful width is 1.4°
      • average angle of 3°, but may be as low as 2.5° or as high as 4°
      • calibrated out to 10 nm from threshold, but reception is common further false glideslope forms at approximately 12° above the horizontal

        • this is the danger of intercepting from above
        • reverse sensing makes it somewhat apparent
      • intercepts marker beacons at 1400 feet HAT (OM) and 200 feet (MM)
      • crosses the threshold at approximately 50 feet
    3. marker beacons, IF 324

      • essentially an extremely low power ADF; designed to create an elliptical 'fan' (alternate name: fan marker) that is 2400 feet wide and 4200 feet long at 1000 feet above the antenna

        • use 'low' setting if available to detect marker beacons
      • outer marker (OM, ––, blue) is 4 to 7 nm from the threshold, and roughly 1400 feet HAT
      • middle marker (MM, –·–·–·, amber) is .6 nm from the threshold, 200 feet HAT, and indicates the DH and/or missed approach point
      • inner marker (IM, ······, white) is used on higher-precision (Cat II or greater) approaches and is between the MM and threshold, usually indicating an altitude of 100 feet or less

        • designates a specific decision height for that approach
      • backcourse marker (BC, ··  ··) is used to indicate a backcourse localizer FAF, where the descent is to be commenced
      • some marker beacon sites are colocated with compass locators, which are much higher powered and are terminal area NDBs
    4. approach lighting systems, IF 326
    5. runway markings
  • DME, radar, and LOM beacons can also be included in or required for ILS approaches
  • Variations on the localizer exist - IF 339-340

    • SDF - simplified directional facility; 6° or 12° wide
    • LDA - localizer directional aid; not aligned with the runway

Distance Measuring Equipment, IF 207 & IFH 7-13

  • Determines the slant range to the DME station

    • hypotenuse of the aircraft's actual distance and altitude
    • errors show up when extremely close
    • accurate as long as we are 1 nm away for every 1,000 ft of altitude above the station
  • Display unit shows distance in nm; our units also show ground speed and eta

    • inside of 200 nm, with an accuracy of less than 1/2 nm or 3% of the distance
  • Airborne transmitter (interrogator) broadcasts a signal

    • this is picked up by ground stations (transponder)
    • ground station replies to the aircraft
  • The DME hardware measures the time lapse
  • Ground stations are limited to approximately 100 aircraft at a time

    • UHF equipment, generally colocated with a VOR as a VOR/DME

DME Arcs - IF 553

  • "Arc northwest" - what does the phraseology mean?
  • Used as a transition to a VOR or ILS approach

    • essentially, we're drawing a constant-radius circumference
    • flown as a series of short, straight segments with heading changes
  • Prior to entry, tune and identify

    1. set up the OBS to read TO the station (with two, set the second to the final approach course) - make sure we're not using the GPS
    2. 1/2 to 1 nm from the arc, turn in the arc direction - generally 90° from the intercept heading

      • GS/200 gives the radius of a standard rate turn; 100 knots is .5 nm, 200 is 1 nm
    3. twist the OBS 10° ahead - we want to cross the next radial
    4. as the CDI centers, turn 10° in the direction of the arc
    5. twist the OBS another 10° ahead and repeat
  • when two dots from centering on the approach course, or when crossing the lead-in radial (on some approaches), start a standard rate turn to intercept
  • Illustrate with Tim's simulator, using approach terminology
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