FAA Instrument Rating Airplane (IRA)Navigation SystemsMedium
A pilot is flying an ILS approach and notices that the glideslope indicator is continuously fluctuating, making it difficult to maintain a stable vertical path. The localizer is steady and within limits. What is the most likely cause of this glideslope fluctuation?
- AInterference from other aircraft on the localizer frequency.
- BA malfunction in the aircraft's VOR receiver, affecting glide slope interpretation.
- CReflected signals from objects on the ground, such as hangars or terrain.
- DHeavy precipitation in the vicinity of the glideslope transmitter.
Show answer & explanationAnswer & explanation
Correct answer: C. Reflected signals from objects on the ground, such as hangars or terrain.
ILS glideslope signals are susceptible to 'roughness' or fluctuations caused by reflections from objects on the ground, such as hangars, terrain, or moving vehicles. These reflections can create false or fluctuating signals, making it difficult for the pilot to follow the true glideslope.
Why the other options are wrong
- A. Localizer and glideslope operate on different frequencies; localizer interference would not directly cause glideslope fluctuation.
- B. The VOR receiver is separate from the ILS system; a VOR malfunction would not affect the glideslope indicator.
- D. While heavy precipitation can affect radio signals, glideslope fluctuations are more commonly attributed to ground reflections rather than direct weather interference with the signal itself, especially if the localizer is unaffected.
ILS Glideslope Anomalies
ILS glideslope signals can be affected by reflections from ground objects or terrain, leading to fluctuations or 'roughness' that make it difficult to maintain a stable vertical path.
- Reflections are a primary cause of instability.
- Can lead to false glideslopes.
- Requires careful monitoring and pilot intervention.
Memory trick: Glide Slope: Ground Reflects, Rough Ride Results.