American Federal regulators want to fine Boeing $19.68 million for installing sensors that the agency says might not have been compatible with a system pilots use to see instrument readings. The sensors make it possible for pilots to use “heads-up” systems that display altitude, airspeed and other information on a glass screen in front of them so they don’t have to look down, away from the windshield.

The Federal Aviation Administration notified Boeing of the proposed fine on Friday and gave the company 30 days to pay or respond to the allegations. The sensors at issue were installed on nearly 800 planes.

Boeing said it has co-operated with the FAA. It said the matter involves documentation of parts and is not a safety issue.

The FAA said the sensors were installed in heads-up display systems on 618 Boeing 737 NG planes between June 2015 and April 2019 and on 173 Boeing 737 Max jets from July 2017 until March 2019.

The heads-up systems involved are made by Rockwell Collins. Boeing failed to verify that the sensors were listed as interchangeable, according to Friday’s letter from a lawyer in FAA’s enforcement division near Seattle to Lynne Hopper, Boeing’s vice-president of engineering for commercial airplanes.