Photo: G550 N535GA Aerial view of accident site, from NTSB Accident Docket
Poor pilot decision making is not a conscious choice, of course. Pilots don't set out to make poor decisions but there are some pilots who are poor decision makers and are predisposed to making poor decisions. In mishap report after mishap report you can almost predict the pilot in question is going to screw things up. When you pair two such pilots together, even a minor problem can become something much more severe. This is just such a mishap.
This mishap should not have occurred, it could have been handled by any two G550 pilots straight out of school willing to approach their jobs with a little more professionalism.
One of the problems with pilots who have little oversight is they start to rationalize all of their actions and soon their subconscious agrees with everything they do. They will lose that "voice in their head" that checks everything they do. See: Normal Procedures & Techniques / Decision Making.
What follows are quotes from the sources listed below, as well as my comments in blue.
Figure: N535GA Route of Flight, from flightware.com
[NTSB Factual Report, pg. 1]
This aircraft was owned by Paul Allen of Microsoft and just had the interior installed prior to final delivery. The pilots in question were full time pilots employed by Gulfstream. Both were former military fighter pilots but there is no evidence they had been formally trained as test pilots.
Figure: G550 Hydraulic System Schematic, from G550 Quick Reference Handbook, pg. EE-2.
In a G550 this is more serious than the name of the abnormal would indicate. The chances of hydraulic quantity being merely a little low is remote. The fluid is under high pressure and the system is pretty tight, you can go years without having to service it. If the fluid is low, it is probably on its way to zero. To make matters worse, the left system runs most of the airplane. There are two back ups, but both these can go south depending on where the leak is. The Power Transfer Unit uses right system pressure to drive left system components, but will not work if left system fluid is gone. The auxiliary pump can drive just about everything you need to land the airplane, but if the leak is in one of these components you will be relying on nitrogen to extend the landing gear (works well), the flaps will be inoperative (not a big deal with this airplane), and will be left with whatever pressure you have with the brake accumulator. The airplane can be stopped, but you need to have your act together to do this.
This makes absolutely no sense: he decided to land because he was low to the ground and knew he had a significant hydraulic leak in the landing configuration. First of all, they had planned on full flaps and didn't have that, they were not in the planned landing configuration. Secondly, and more important, they were in the process of losing the hydraulic system responsible for their brakes. Every Gulfstream pilot since the GII knows that if you have a leak in the left system (called the "combined system in the GIV and earlier) you are at risk losing the auxiliary system too.
We also have a serious CRM issue here. The checklist directs a longer runway and they had a longer runway at their disposal. The weather was good, the winds actually favored the longer runway, and they didn't have a fuel issue. The PF had no compelling reason to ignore the PNF's recommendation to go around. Further, the PNF should have been more insistent. More about this: Procedures & Techniques / Crew Resource Management.
One of the complicating factors is that the hydraulic systems synoptic page can show varying amounts of fluid in the left system, which shares fluid and sits on top of the auxiliary system. Once the left system is depleted, the auxiliary system synoptic either shows full or empty, there are no intermediate indications. The auxiliary system could be intact or it could very well be the source of the leak, you don't know.
The G550 checklist has since been changed to leave the auxiliary pump alone at the point, assume it will exhaust the fluid, and land with it in the AUTO position. It will give the pilot full antiskid braking until it exhausts itself, at which point the pilot will have an easier time with the emergency brake. The fact both pilots indicated no concern about losing the normal spoilers, brakes, and nose wheel steering indicates a profound lack of systems knowledge.
This is a common sensation when the automatic ground spoilers are inoperative.
Here again a serious CRM issue. It is usually fool hardy to decide to balk a landing once the thrust reversers are out. It will take time to stow the reverser and while you are doing that it is headed for idle, further delaying spool up time for the go around.
Examination of the wreckage revealed the brake accumulator still had a full charge of 3,000 psi. The emergency brake could have brought the airplane to an uneventful stop had it been used early after landing.
Photo: G550 N535GA Landing incident runway (Gulfstream submission), from NTSB Accident Docket
It appears the leak was in the nose wheel steering system which would cause the left system fluid to be depleted and the auxiliary system to be depleted once the auxiliary pump was activated. (That would happen once the switch was turned on or if the AUTO position once any wheel brake pedal exceeded 10° deflection.)
Once the first indication of hydraulic system failure had occurred, the prudent decision would be to go around and carefully consider the ramifications. It is likely the pilots would have then realized a longer runway was necessary and the likelihood of losing brakes would require the emergency brakes.
Even after all these improper decisions, once the thrust reversers were deployed a safe stop could have been achieved using the emergency brakes. The "go" and then the "no go" indecisions sealed the aircraft's fate.
[NTSB Factual Report, pg. 1j] The National Transportation Safety Board determines that the probable cause of this accident was the captain's decision to attempt a go-around late in the landing roll with insufficient runway remaining. Contributing to the accident were (1) the pilots' poor crew coordination and lack of cockpit discipline; (2) fatigue, which likely impaired both pilots' performance; and (3) the failure of the Federal Aviation Administration (FAA) to require crew resource management (CRM) training and standard operating procedures (SOPs) for 14 CFR Part 135 operators.
Gulfstream G550 Quick Reference Handbook, GAC-AC-G550-OPS-0003, Revision 27, 24 July 2008
NTSB Factual Report Aviation, CEN11FA193, N535GA, 02/14/2011, Appleton, WI.