The student should gain an understanding of DME operation and the instrument landing system, and be able to apply that knowledge to flight scenarios.
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:
- 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
- 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
- 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
- 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
- approach lighting systems, IF 326
- runway markings
- localizer provides lateral (directional) guidance
- 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