OBS  FR TO  N  E  S  W  3  33  24  21  15  12  30  6  NAV 1  Outer Marker Beacons  Middle Marker Beac ons OBS  FR TO  N  E  S  W  3  33  24  21  15  12  30  6  NAV 1  4100  4000  3900  3800  3600  3500  3400  3300  3200  3100  20 80 4000 4400  4300  4200  4100  4000  3900  20 80 4000 4000      Figure 7-35. Glide-slope receiver indic ations and aircraft displacement.  3600  Figure 9-39. A GS receiver indication and aircraft displacement. An analog system is on the left and the same indication on the Garmin  PFD on the right. Some marker beacon receivers, to decrease weight and cost,  are designed without their own power supply. These units  utilize a power source from another radio in the avionics  stack, often the ADF. In some aircraft, this requires the ADF  to be turned on in order for the marker beacon receiver to  function, yet no warning placard is required. Another source  of trouble may be the “HIGH/LOW/OFF” three-position  switch, which both activates the receiver and selects receiver  sensitivity. Usually, the “test” feature only tests to see if  the light bulbs in the marker beacon lights are working.  Therefore, in some installations, there is no functional way  for the pilot to ascertain the marker beacon receiver is actually  on except to fly over a marker beacon transmitter and see if  a signal is received and indicated (e.g., audibly, and visually  via marker beacon lights). The simplified directional facility (SDF) provides a final  approach course similar to the ILS localizer. The SDF course  may or may not be aligned with the runway and the course  may be wider than a standard ILS localizer, resulting in less  Simplified Directional Facility (SDF) Operational Errors 3500  3400  2.  Disorientation during transition to the ILS due to poor  planning and reliance on one receiver instead of on all  available airborne equipment. Use all the assistance  available; a single receiver may fail. 3300  3.  Disorientation on the localizer course, due to the first  error noted above. 3200  3100  4.  Incorrect localizer interception angles. A large  interception angle usually results in overshooting  and possible disorientation. When intercepting,  if possible, turn to the localizer course heading  immediately upon the first indication of needle  movement. An ADF receiver is an excellent aid to  orient you during an ILS approach if there is a locator  or NDB on the inbound course. 5.  Chasing the CDI and glidepath needles, especially  when you have not sufficiently studied the approach  before the flight. 1.  Failure to understand the fundamentals of ILS ground  equipment, particularly the differences in course  dimensions. Since the VOR receiver is used on the  localizer course, the assumption is sometimes made  that interception and tracking techniques are identical  when tracking localizer courses and VOR radials.  Remember that the CDI sensing is sharper and faster  on the localizer course. 9-42