In high-power HF radio operation, which of the following is a potential effect on the RADALT indicator?

• A. No effect
• B. Fluctuations in RADALT indications
• C. RADALT becomes unreadable
• D. RADALT reads higher than actual altitude

Answer: (B)

High‑power HF operation can inject RF noise and EMI into sensitive avionics like the radar altimeter. The RADALT system relies on a stable RF signal and clean returns to measure altitude above terrain. When a strong HF transmitter is active, the RF energy can couple into the RADALT receiver or its cables and antennas, creating noise and potential spurious echoes. This makes the detected range data wander from sample to sample, producing fluctuations in the RADALT indications rather than a steady, accurate reading. In typical conditions, you’ll see jitter or instability as the system tries to lock onto the true return, instead of a constant bias or a complete unreadable state.