- There was no procedural or flight rule requirement for the mission specialists seated on the middeck to be seated at a particular time.
- The Orbiter g-loading during entry was moderate (~1.6 g's max) and increased slowly.
- The Orbiter seats were not equipped with tray tables, but did incorporate a five-point restraint harness.
Procedural Details and The Lack Thereof
To explain the shuttle entry procedures and their time references, some background information must be covered.
Two reference times were used in the procedures. One timeline counted down to the ignition of the deorbit burn, written TIG, and the other timeline counted down to Entry Interface, written EI. EI was defined as the Orbiter reaching an altitude of 400,000 feet. This happened about 30 minutes after completion of the deorbit burn.
At any given point during a shuttle mission, one particular checklist was the "controlling document". During the first part of the countdown to TIG, the controlling document was the Deorbit Prep Checklist. Thirty minutes prior to the burn, the Entry Checklist became the controlling document, and remained the controlling document for the remainder of the mission. The controlling documents and their times of applicability are shown in this graphic. I have highlighted the ones of interest to this answer.
The Deorbit Prep Checklist contained a useful summary timeline of the countdown to TIG. I show an excerpt here with references to seat ingress annotated. Note that this timeline, although found in the Deorbit Prep Checklist, shows events extending past the time that this is the controlling document.
However, only the flight deck crewmember seat ingresses are ever specifically called out in the checklists.
- The commander and pilot seat ingresses are called out in the Deorbit Prep Checklist at TIG-00:58
- The mission specialists seated on the flight deck (MS1 and MS2) seat ingresses are called out in the Entry Checklist at TIG-00:25 (This differs from the time line given in the Deorbit Prep Checklist!)
- The seat ingress times are never specifically called out for mission specialists seated in the middeck.
In practice, shuttle mission commanders had a great deal of leeway in how to organize the "housekeeping" portions of their misson. While the "systems" milestones were tightly structured and monitored by the ground (when devices were turned on and off, when data was entered into the onboard computers, etc); activities such as donning or doffing of suits, stowing and deploying crew seats, ingressing and egressing those seats, etc; could be arranged by the commander to fit their preferences and the needs of their particular mission.
Here is an example of a such a customized deorbit prep timeline from a mission that I supported. I have removed names and mission designations and highlighted areas pertinent to this question.
Note that this particular commander suited up and and ingressed their seat prior to the start of the deorbit prep timeline, hours before the checklist called for it.
Bottom line, the mission commander determined when the middeck mission specialists must strap in.
The Acceleration Environment of the Orbiter During Entry
After the deorbit burn was executed, there was an approximate 30 minute period of free-fall until EI. After EI, the acceleration built up at a moderate rate, taking ~15 minutes to reach 1 g, and peaking at only about 1.6 gs.
The Absence of Tray Tables
The seats utilized by mission specialists in the orbiter were foldable for stowage. This was done after ascent to make room in the cabin. They were not equipped with tray tables, but did incorporate five-point safety harnesses.