VOR/ILS Bearing Error After Nav Antenna Replacement

The two errors point away from a simple indicator fault. A faulty CDI could cause one error, but it is less likely to produce a consistent VOR bearing error and a separate localizer centring error immediately after an antenna and connector change. The common element is the nav antenna, its coax, the connectors, or the test setup.

Possible causes include:

• Wrong antenna type or a VOR/ILS antenna with a damaged or incorrectly connected feed.
• Coax damage, a poor shield connection, or a connector with a deformed centre pin. At VOR frequencies the signal is relatively low, so a partial shield open can cause bearing errors rather than a complete loss.
• Poor antenna bonding or a changed ground plane affecting the VOR bearing.
• Test-set calibration, incorrect frequency selection, or a test setup that is affected by nearby structures and multipath.
• Receiver configuration or a faulty nav receiver, although this is less likely if both VOR and localizer errors appeared at once.

I would first verify the test set and repeat the ramp test at a different location, then inspect the antenna part number, connectors, coax, and bonding. A voltmeter and visual inspection will not prove RF performance, so I would use the nav test set and, if available, a coax/connector check. I would not adjust the indicator to hide the error before the RF path is verified.

Not necessarily. A ramp test is a useful ground check, but it is conducted in a reflective environment and may not reproduce the signal environment in flight. The manual specifically requires a ramp test after antenna replacement and refers to a flight check where required. With an 8-degree VOR error and a one-dot localizer offset, the ramp result is already outside what I would expect for a satisfactory installation.

I would complete the ground checks and correct any obvious antenna, coax, connector, or bonding fault. If the ramp test then meets the manual tolerance and the manual does not require a flight check for that repair, I would consider release according to the approved data. If the error persists or the manual requires a flight check after antenna replacement, I would arrange the flight check before IFR release. VMC weather does not remove the requirement, because the aircraft is intended for IFR operations and the nav system is part of that capability.

A flight check should be performed by an appropriately qualified provider against the required tolerances. The fact that the operator wants to depart does not replace the manual requirement.

I would record the original discrepancy, the antenna and connector replacement, the coax type and length, the ramp-test setup and frequencies, the VOR error before and after, the localizer deviation before and after, any bonding or connector corrections, and the flight-check result if one was required. I would reference the maintenance manual tolerance and the flight-check requirement.

If the operator asked me to adjust the CDI to remove the offset, I would refuse unless the approved adjustment procedure and the test results showed that the indicator was the actual cause. Adjusting the indicator to compensate for an antenna or coax fault would mask a real signal-path error and could leave the aircraft with unreliable navigation. The correct sequence is to verify the test equipment, inspect and correct the installation, repeat the ramp test, and carry out the required flight check. If the system cannot be brought within tolerance, the aircraft should not be released for IFR navigation on that system.