Spaceflight Now

Telemetry shows autopilot on through last transmission
Posted: March 9, 2003

Ongoing analysis of the final two seconds of telemetry from the shuttle Columbia during re-entry Feb. 1 shows the doomed ship's fuselage, crew module, right wing and right-side rocket pod were essentially intact 32 seconds after the commander's final transmission and that the orbiter's digital autopilot was still flying the spacecraft. A computer alarm generated in that final two seconds of data suggests one of the pilots' joystick hand controllers may have been briefly engaged, but the autopilot was never deactivated before contact was lost.

By that point, however, there was nothing the crew could have done to stop the quickening disaster. The telemetry shows Columbia's left wing and left-side orbital maneuvering system rocket pod were either gone or severely damaged, the ship's hydraulic system was empty, its flash evaporator cooling system was in shut down and multiple computer alarms were being generated because of lost data from the orbiter's left side orbital maneuvering system rocket pod.

Guidance and navigation data show the shuttle was in an "uncommanded" orientation, yawing rapidly to one side, presumably toward Earth, in what may have been the start of a banking tumble. The yaw rate - a measure of how fast Columbia's nose was swinging to one side - was at least 20 degrees per second, the maximum value the sensors are designed to measure. The actual yaw rate may have been higher.

That same two seconds of "ratty" telemetry shows one of the cockpit's two rotational hand controllers, or joysticks, may have been briefly engaged as early as 9:00:01.7 a.m. That was nearly 30 seconds after commander Rick Husband's final interrupted transmission to Houston at 8:59:32 a.m. But the timing is uncertain because of the duration of software-driven data sampling rates. The final bit of telemetry, however, shows Columbia digital autopilot was still in control when the flow of data finally ceased and that the hand controller was in its normal "centered" position. As such, it is not known whether the RHC was bumped inadvertently by the commander or pilot or whether one of them intentionally gripped the stick with thoughts of taking over manual control.

It is also possible both scenarios are false. Data from this final two-second period was corrupted by transmission errors and as such, it is subject to error or misinterpretation. But it was included as part of revision 14 to NASA's STS-107 Mishap Investigation Master Timeline, a revision that was never released, sources say, because an updated version is in the final stages of preparation. At least some of the data in revision 14 may have been corrected, eliminated or expanded in the latest revision. This status report will be updated as warranted when the new revision is released.

In any case, engineers now believe the main body of the spacecraft did not begin breaking up until nearly 20 seconds after the final two-second burst of telemetry. Vehicle breakup was preceded by the separation of at least three major pieces of debris beginning around 9:00:02 a.m., at almost the same instant the final two seconds of telemetry began flowing back to Earth after a 25-second data dropout.

The timeline also includes more than a dozen "debris shedding" events recorded by observers on the ground as Columbia crossed above California and the southwestern United States. The first such confirmed instance of debris falling away from the shuttle occurred around 8:53:44 a.m., 18 seconds after the spacecraft passed above the California coast just north of San Francisco.

But it is the orbiter's final minutes that grip the imagination as the data plays out with a sort of slow-motion inevitability.

At 8:59:32 a.m., Husband called Houston, presumably to report or confirm a computer fault message showing lost pressure from both left-side main landing gear tires.

"Roger, uh, buh..." he radioed, but he was cut off. The "buh" may have been the beginning of the word "both" or possibly "before." There is no way to know, but engineers now say that final transmission was interrupted not because of the mounting problems on board the shuttle, but because the line of sight between a forward antenna cluster and the NASA communications satellite then in use was blocked by Columbia's vertical stabilizer and aft engine compartment.

In any case, raw data continued to flow for another five seconds before it, too, was interrupted by antenna blockage. A master alarm sounded in the cockpit during those five seconds, computer messages were generated that noted problems with the shuttle's flight control system and an electrical system powered down.

Toward the end of that five seconds, the aerodynamic "sideslip" being experienced by Columbia began reinforcing the aerodynamic drag already pulling the ship to the left because of problems with the left wing. At 8:59:36 a.m., the digital autopilot, struggling to keep the shuttle properly oriented, was forced to drop the left wing to compensate for the increasing aerodynamic forces acting on the craft.

Less than a second later, the autopilot commanded a right-pointing yaw thruster to fire to help counteract the growing yaw motion. Two other jets already were firing and a fourth was commanded on less than a second after the third.

And then, at 8:59:37.396 a.m., telemetry was cut off. For the next 25 seconds, no telemetry was received due to antenna blockage. A final two-second burst came down beginning at 9:00:02.660 a.m. The data were garbled and some of the readings are not clear cut. But engineers were able to recover computer message stored in "buffers" that provide at least some insight into what was going on during that preceding 25 seconds.

The shuttle's primary flight computers generated a "roll reference" fault message around 8:59:46 a.m. Six seconds later the first in a series of left OMS rocket pod alarms was generated by the shuttle's flight computers. In the final two seconds of telemetry, additional OMS pod alarms were generated, along with an alarm indicating an apparent short circut.

In the final two seconds of telemetry, it appears Columbia's hydraulic power units were operating relatively normally, the shuttle's water spray boilers were cooling the hydraulic system lubrication oil, the main propulsion system and aft engine compartment were intact and the shuttle's three electricity generating fuel cells were operating.

In addition, Columbia's communications equipment and navigation systems were still functioning and temperature readings were still being received from the ship's vertical stabilizer, body flap, main engine compartment and the right wing.

But all three hydraulic systems were operating at zero pressure with empty hydraulic fluid reservoirs, presumably because of severe left wing damage that ruptured the triply redundant hydraulic lines. Temperature readings from the left wing elevon actuators were absent, the shuttle's flash evaporator cooling system had apparently shut down and the majority of the sensors in the left orbital maneuvering system rocket pod were either disabled or showing abnormal readings. Major electrical problems were beginning to develop and elevated temperatures were noted on the belly of the shuttle and along its left side.

Guidance and navigation data suggests vehicle was in an uncommanded attitude and was exhibiting "uncontrolled rates," the timeline states. "Yaw rate was at the sensor maximum of 20 degrees per second. The flight control mode was in AUTO. Based on the nominal and off-nominal system performance described above, it appears that the fwd/mid/aft fuselage, right wing and right pod were still intact."

The final bit of telemetry from Columbia showed a computer fault message regarding the rotational hand controller. While the "digital autopilot roll stick function" was initialized, "available vehicle data indicates the RHC was in detent (in the normal centered position) and (the) digital autopilot was in AUTO."

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