Figure: N777TY, from Airliners.net
According to the accident report this mishap was caused by by mechanics who forgot to remove popsicle sticks used during routine maintenance and by Gulfstream who should have had a better method to disable the weight on wheel switches.
Those points are true, but they didn't cause the mishap. The mishap was caused by pilots who failed to perform a complete external preflight, failed to correctly analyze clearly presented crew alerting system warnings, and failed to execute the proper emergency procedure checklists. Additionally, they failed to take advantage of several commonly accepted techniques, any one of which would have prevented this aircraft from being destroyed.
In the words of the captain, these pilots "blew it." I think they did. But I'm not sure they were bad pilots, they made it to the premier jet in corporate aviation. There weren't many GV pilots in 2002 and they had to be of a special cut to make it to that jet. But even the best pilots can become complacent and complacency shows at every phase of this mishap. I think there are several lessons here for all pilots, Gulfstream or not:
What follows are quotes from the sources listed below, as well as my comments in blue.
The main landing gear weight on wheel (WOW) switches are switches with a roller on a cam. When the aircraft is airborne the cam rotates to a low point and the switch extends. The mechanic needed both main landing gear WOW switches to be in the ground mode while the aircraft was on jacks so he placed the wooden sticks between the cam and the switch on both gear, forcing it into the "ground mode."
The wooden sticks did not have warning flags attached. Gulfstream now has purpose-made wedges with large red "Remove before flight" banners.
The GV Airplane Flight Manual, §2-01-20, steps 53 and , require: "MLG WOW Switch . . . CHECK. From the earliest GII Gulfstream pilots are taught these switches are about the most important item on an external preflight. The ground spoilers are tied to them and having a ground spoiler deploy while in flight can be catastrophic. The sticks should have been obvious. (A mechanic spotted them immediately upon coming to the wreckage after the crash.)
"No lock release" refers to the landing gear lever lock which prevents the gear handle from being selected "up" when the aircraft is in the ground mode. On this airplane, the handle is an electrical switch with no mechanical linkage to the gear. Gulfstream pilots instinctively raise the gear and disarm the ground spoilers, but it is easy to have this pattern interrupted if the gear handle doesn't move. A better technique is to disarm the ground spoilers before attempting to retract the landing gear. More about this: G450 Normal Procedures & Techniques / Takeoff / After Lift Off.
Step one of that checklist in GV Quick Reference Handbook, Page EG-3, is "GND SPLR Switch . . . OFF.
A test of the system, duplicating the conditions, revealed the CAS messages would have been GND SPOILER (red); WOW FAULT (amber); and WOW FAULT (blue). The checklist they selected would not have been called for. All three of the indicated checklists directed the ground spoilers to be turned off.
This was not a normal situation, to be sure. But one of the practiced callouts in this airplane is "Three green, four in the air" prior to turning the ground spoilers on. The three green are the nose, left main, and right main down and locked indications. The "four in the air" refers to the three landing gear WOW switches and a fourth "combined" WOW which compares everything to the radio altimeter and speed of the aircraft as a cross check. After a while these rote call outs can lose their meaning and perhaps that was true here. But the reason Gulfstream V series pilots use the "three green, four in the air" callout is as a prerequisite to turning the ground spoilers on. The first officer would not have seen dashes where the left and right WOW switches were indicated on the synoptic page, he would have seen the letter "G" to mean the airplane thought it was on the ground. Years later the first officer was teaching at a simulator school where instructors taught the fourth WOW component, known as the "Combined WOW," would prevent ground spoiler activation in the air. That is not true. All it does is warn you there is a problem. With both main landing gear WOW indicating "G" all they needed for ground spoiler deployment was for the throttles to retard to idle.
[GV Aircraft Operating Manual, §2A-27-70, ¶2.B.(1)] With the GND SPLR switch selected to ARMED, the ground spoiler control system will extend all six spoiler panels in unison to 55 (±4) degrees when the following parameters are satisfied:
[GV Aircraft Operating Manual, §2B-05-40, ¶5] The retard mode is used to establish a fixed rate of throttle retardation to the idle thrust position during the aircraft flare on landing. The retard mode is used for AP/FD coupled and non-coupled approaches. It is activated based on radio altitude < 50 feet and gear down.
It appears the autothrottles were not being used and the thrust was "chopped" abruptly at 57 feet, satisfying all the requirements for ground spoiler deployment and resulting in the very hard landing. Had the autothrottles been used, it is unlikely the throttles would have hit idle until very close to the ground and that, by itself, could have avoided the crash.
[NTSB Accident Summary] The flight crew's failure to follow preflight inspection/checklist procedures, which resulted in their failure to detect wooden sticks in the landing gear weight-on-wheel switches and their failure in flight to respond to crew alert messages to disarm the ground spoilers, which deployed when the crew moved the throttles to idle during the landing flare, causing the airplane to land hard. Contributing to the accident was maintenance personnel's failure to remove the sticks from the weight-on-wheels switches after maintenance was completed.
Gulfstream GV Airplane Flight Manual, Revision 30, 13 May 2008
Gulfstream GV Aircraft Operating Manual, GAC-AC-GV-OPS-0002, Revision 30, May 13, 2008
Gulfstream GV Quick Reference Handbook, Revision 30, June 26, 2008
NTSB Aircraft Accident Summary and Narrative, MIA02LA060, Gulfstream G-V, N777TY, February 14, 2002