United 93 ACARS: Uplink & Downlink Acknowledgement, Simply Explained

Regarding the crucial messages to United 93 (UAL 93) after the Shanksville crash, Warren Stutt stated in 2012, that the aircraft did not receive ACARS messages #18 and #19, because they were not “acknowledged”, by a DLBLK downlink acknowledgement.

The ARINC file shows that there are no type DLBLK blocks and therefore no ACARS messages were received from UAL93 after the official time of the crash 10:03 a.m.

Stutt seems to believe that every received uplink (ULBLK) is inevitably acknowledged by the aircraft, so if a message is not acknowledged, it has not been received by the aircraft, however, this is not true. An aircraft may receive an uplink, while ground control does not receive the acknowledging downlink (DLBLK).

In the context of ACARS, “not acknowledged” is not equivalent to “not received.” Those are two different claims.

Here’s why…

An ACARS uplink typically follows this sequence:

1. Ground system transmits an uplink message.

2. The aircraft’s ACARS unit receives it.

3. The aircraft sends a downlink acknowledgment (ACK).

4. The ground network receives that ACK and records the successful transaction.

If step 4 fails—for example because of radio interference, loss of communications, equipment failure, or the aircraft ceasing transmission—the ground station may have no record of the acknowledgment, even if the aircraft actually received the message.

An important technical distinction in how ACARS works…

The key issue is the difference between:

1. The aircraft receiving an uplink, and

2. The ground station receiving the aircraft’s acknowledgement.

Those are not the same event.

A DLBLK (”downlink block acknowledgement”) is transmitted by the aircraft after it successfully receives an ACARS uplink. But the fact that ARINC never logged the DLBLK does not logically prove the aircraft never received the uplink. It only proves that ARINC never received the acknowledgement.

I hope this simplifies the “uplink” and “downlink” ACARS acknowledgements.

Finally,

I shall be posting one last article, which will be a critical analysis of the International Center For 9/11 Justice - Manual or Automated, Aircraft Flight Path Study, which they recently released, where they concluded that the aircrafts that hit the WTC buildings and Pentagon were automatedly guided into the their targets.

The study itself began from the outset with a myopic intent to offer two choices “Manual or Automated”, while ignoring all other critical evidence, which proves otherwise. This has been promoted by Redacted and UK Column, who I consider to be controlled alternative media. Neither Redacted or UK Column have offered the opportunity of an open Podcast for researchers such as myself to scrutinize the study, which in my opinion speaks volumes!

I hope to follow-up my critique of the study with a discussion about it with 9/11 Revisionist. I did offer the author of the study Piers Robinson, and also Ted Walter, an invitation to discuss the study, however this has not been taken-up as yet. I believe 9/11 Revisionist has also e-mailed them both to see if they would like to have an open discussion, and allow questions I have surrounding the study.

Thanks for reading and caring!