The following interview has been conducted by the FBI on 2/15/2002. I have transcribed the relevant parts. The source URL is here:
Knerr provided information related to the printout of text messages transmitted to and from UA Flight 93 on 09/11/2001 through the AIRCRAFT COMMUNICATION ADDRESSING AND REPORTING SYSTEM (ACARS). Knerr identified this system as one means of communication that is utilized between aircraft and commercial carriers, like UA, to transmit text data. Knerr related that data is either uplinked to the aircraft from fixed communication centers or downlinked from the aircraft to receiving communication centers. Knerr explained the uplink and downlink references on an ACARS message. DLBLK refers to downlink while ULBLK refers to uplink.
These references also identify that a ACARS message has been received by its sender, either ground communications or the aircraft. In the final moments, at 10:12 AM EST, of UA Flight 93's flight, ACARS messages were being sent from ground communications but were not being received. This was causing the ACARS messages to be rejected. Knerr advised that Flight 93's low altitude may have caused this dilemma or the fact that Flight 93 had already crashed at the time messages were sent.
Knerr further advised that AERONAUTICAL RADIO INCORPORATED (ARINC) serves as the network administrator for the communication data. According to Knerr, this is important to remember when reviewing data messages because uplink and downlink times may show a time delay when compared. This is caused by multiple processing of multiple ACARS messages through the ARINC network at the same time. ARINC serves a substantial portion of the commercial airline industry operating within the United States. Depending on the time of day or region the country that an aircraft may be operating over, ACARS traffic can be delayed.
Knerr advised that when an aircraft downlinks data to communication towers it does so by sending out messages over a large geographic area that it is flying. Depending on the area of the country, more than one communication tower may receive the aircraft's message. Knerr pointed this out to be the case during Flight 93's flight over New Jersey, Pennsylvania, and Ohio on 9/11/2001.
In addition, Knerr pointed out that on two separate instances during Flight 93's flight on 9/11/2001, prior to crashing in Pennsylvania, two alphanumeric messages were sent from the aircraft to ground communication. These messages were routine maintenance messages that are sent in order to identify the current state of mechanical operations onboard an aircraft during flight. It allows for specialists to view specific mechanical data onboard an aircraft while it is in operation. Knerr advised that these messages, once deciphered, may contain information that would demonstrate that the aircraft was being operated in a manner that was adversely affecting the performance of the aircraft. The analysis of these messages is forthcoming.
The ACARS report provided by Knerr will be maintained in the 1A section of this file. Moreover, this data will also be included on the Newark Investigation Summary, Flight Operations Time Line.