Radar Altimeter Retrofit Under the FAA Rule: What Changes on the Aircraft

What is actually changing on the aircraft?

In most cases, the transceiver. The rule sets a performance requirement for the altimeter: it has to keep working from the ground to 500 ft AGL in the interference environment the rule defines. It does not prescribe an installation. The FAA regulatory analysis assumes the retrofit is a transceiver swap, plug-and-play on most aircraft, because replacing the antenna or wiring would greatly increase completion time and cost.

Do the antennas and coax stay?

That is the FAA’s expectation. The rule states that previously installed antennas will not require requalification where the transceiver demonstrates the frequency selectivity of the interference tolerance mask. In other words, the filtering burden sits in the box. Whether a given unit carries that burden is a question for its manufacturer, so get the answer in writing before assuming the antennas stay.

We added filters for the current directives. Does that carry the aircraft to the new rule?

Not on its own. The new mask goes further than the directives in force: it adds a requirement in the 4170 to 4180 MHz range and carries requirements up to 5.6 GHz, with a six decibel margin above the maximum environment the FCC permits. The rulemaking record reflects industry agreement that no altimeter in service meets it today. Treat any existing filter as a starting point to evaluate, not as a finished retrofit. An external filter also stays on the aircraft as space, weight, connections and approval paperwork you will be working around.

Can the existing unit be upgraded instead of replaced?

Depending on the manufacturer, yes. The FAA confirmed that modifying a design already authorized under TSO-C87 or TSO-C87a qualifies as a minor change under the existing authorization, where the scope does not require a substantially complete investigation. That is what makes a service bulletin upgrade possible. The other routes are a new TSO-C87a authorization, or TSO-C221 once the RTCA SC-239 and EUROCAE WG-119 standards publish, expected in early 2027. What the upgrade involves at the unit level, and whether it is done on the aircraft or on the bench, will be defined in each manufacturer’s service bulletin.

What approval will the install need?

The rule allows an amendment to an existing type certificate or STC, or a new STC, and approval of radio altimeter systems is eligible for delegation to designees and ODA holders. The FAA says the final version of AC 20-199, Installation of an Airborne Low-Range Radio Altimeter System, will be published to support implementation. Until it is, confirm the approval basis for each airframe before you quote: whether an amended STC will cover it, or whether it needs its own approval.

What does the altimeter feed, and what has to match?

The retrofit changes a sensor that other systems depend on. Radar altitude feeds TAWS and GPWS, TCAS, the autopilot and flight director, height displays, and altitude callouts and alerts. Before you commit to a replacement, match the outputs those systems expect: the digital bus, any analog output, trip points and discretes, and the indicator. An interface mismatch turns a transceiver swap into a wiring and panel job, which is the cost the FAA analysis assumes you avoid.

Is anything on today’s installs out of compliance?

No. Altimeters recognized as tolerant under the directives in force, AD 2023-10-02 and the model-specific ADs for transport airplanes and AD 2023-11-07 for rotorcraft, keep operating as they do now. An altimeter that meets the new standard will be recognized as satisfying those directives, and the FAA plans to remove them after the final compliance date of October 31, 2034.

Which aircraft will come through the hangar first?

Part 121 aircraft, and Part 129 foreign carriers with 30 or more seats or over 7,500 lb payload, have the earlier date of December 30, 2030. Aircraft on legacy RA-4000 and RA-4500 units are natural first-wave conversions. Expect Canadian-registered aircraft too: the FAA rule does not apply in Canadian airspace, but Canadian aircraft flying into the contiguous US operate as Part 129 and are caught on the same dates. Transport Canada recognizes altimeters that meet FAA tolerance standards under AD CF-2024-14 and CF-2024-15.

What about rotorcraft?

The same standard applies, but the FAA has not yet confirmed the rotorcraft operational path, including how it treats procedures that depend on altimeter data such as hover and search and rescue autopilot modes and night vision goggle operations. On rotorcraft installs, pay particular attention to those downstream functions when you evaluate a replacement.

What should we record now?

An inventory built at the next visit saves a scramble later. For each aircraft:

• Altimeter part number, mod status and software level
• Antenna type and condition, coax routing, and any filters already installed
• Indicator, and every system the altimeter output feeds
• Current approval basis: STC, field approval or type design
• Registration, and whether the aircraft operates into the contiguous US

For US-registered aircraft, keep that record with the work order. The FCC retrofit rebate will need documentation, and our rule overview covers what is known so far.

Where TERRAIN SERIES fits

Every TERRAIN SERIES model is certified to TSO-C87a and recognized as tolerant under the FAA directives in force and the corresponding Transport Canada directives. Like every altimeter in service, none meets the new mask today. What that means at the install:

• Filtering in the box. Interference mitigation is designed into the unit with custom RF filtering and DSP, not added as an in-line filter
• Legacy drop-in. The RA-4500 MK II is a direct replacement for RA-4000 and RA-4500 units, using existing mounting locations and antennas
• ARINC 429, ARINC 552A, RS-485/422 and RS-232, with analog output on the RA-6500 and RA-5500 for legacy panels
• Panel stays. The RAD-45 indicator is compatible with existing KNI 415/416 installations, so the panel and crew familiarity do not change
• Install setup. Altitude Zero Calibration at installation compensates for cable length and antenna mounting height
• Upgrade path. A service bulletin hardware and software upgrade is anticipated for existing units, sized to fit a scheduled maintenance visit. FreeFlight will apply to amend its existing STCs rather than build new ones, and a next-generation unit is in development for TSO-C221

FreeFlight engineers sit on RTCA SC-239, the committee writing the standard TSO-C221 will draw from, and FreeFlight is listed among the Joint Aviation Community signatories in the final rule. FreeFlight Systems has designed and built avionics for 36 years.