Which radar altimeters are susceptible to 5G C-band altimeter interference?

Question

There is apparently a great deal of consternation over the pending expansion of 5G

The emergency petition claimed that interference from the C-Band "will cause irreparable harm and jeopardize the function of critical aircraft safety systems, which in turn threatens to divert or cancel thousands of flights every day, thus disrupting millions of passenger reservations, causing substantial disruptions for air crews, further interrupting the U.S. and global supply chains, and eroding the safety margin that the industry and the Federal Aviation Administration have worked so hard to achieve."

Apparently the FAA wanted two more weeks to study the issue

As part of this proposal, we ask that your companies continue to pause introducing commercial C-Band service for an additional short period of no more than two weeks beyond the currently scheduled deployment date of January 5. During this time, the FAA will identify the priority airports, issue the required Notices to Air Missions (NOTAMs) and begin approving AMOCs. During this time, the FAA will review information relating to the size of the buffer zone around critical airports and will seek to reduce the size when safely able based on data from aviation manufacturers. The FAA will make every effort to complete this work as expeditiously as possible

AT&T and Verizon rejected that request

The FCC had compelling reasons for [limiting the 5G spectrum being used]. Spectrum interference disputes typically involve simultaneous transmissions on the same frequencies. But radio altimeters do not operate on, or anywhere near, the C-Band frequencies. Rather, they operate in a frequency band (4.2-4.4 GHz) that is separated by at least 400 megahertz from the C-Band frequencies (3.7-3.8 MHz) that AT&T and Verizon will begin using in 2022 and at least 220 megahertz from any C-Band frequency authorized for use in the future. This helps explain why C-Band 5G service and aviation operations already coexist in nearly 40 other countries where C-Band spectrum has been deployed without any negative impact on aviation.

What equipment is actually being threatened by this rollout, that would cause airline companies to file suit over it?

Answer 1

Here are two of the recent statements from the FAA the earlier of which states In 2018, the Air Line Pilots Association raised concerns to the FCC. and goes on to link this statement which references this study and ultimately their concerns were about interference with radio altimeters, with which many aircraft may be equipped. The concern was summed up with:

In 2014, working paper ACP-WG-F30/WP-14, “Preliminary Study into Radio Altimeter Adjacent Band Compatibility, 4 ” was presented at ICAO. This study detailed an analysis of the interference on radio altimetry systems that could be caused by International Mobile Telecommunications (i.e. wireless broadband). The study concluded that, “there is the potential requirement for nearly 64 dB of [Out-OfBand] isolation at the band edge to properly [protect] the receiver from false altitude errors.”

The study is only 9 pages long and the conclusion is quite similarly worded but you should take a look if you are curious.

Lots of modern airframes use radio altimeters and an exhaustive list would be impossible to put together as they can be added to just about any airframe. The simplest answer is "a lot of commercial airplanes". I have even seen some larger GA airplanes equipped with them.

Practically speaking this creates an issue when flying approaches where RA is required. See this article for more info on that.