Because of the unique and challenging flying qualities of the Space Shuttle Orbiter when it returns to Earth as a very heavy glider, NASA has had four Gulfstream II jets that have been modified to train Shuttle commanders and pilots in the complex task of bringing the Orbiter to a smooth landing after reentry. To accomplish this unique task, the Shuttle Training Aircraft (STA) has a number of modifications that set it apart from a standard GII business jet. The most important of these modifications is of course, internal, with the Advanced Digital Avionics System (ADAS) that replaces some of the seats (leaving nine seats) on the right side of the forward cabin. The ADAS takes in variables such as the weight of the Orbiter on return to Earth, runway and direction/elevation of the field in question and then moves the flight control surfaces and throttles of the STA to simulate how the Orbiter would respond to control inputs from its pilots.
The trainee pilot sits in the left side of the STA cockpit which has all the necessary instrumentation and heads-up display (HUD) as used on the Shuttle as well as the Orbiter's left hand stick controller. The front and side cockpit windows on the left side of the STA cockpit are also partially masked to give the pilot flying the simulation the same angular field of view as if he or she were on the Orbiter's flight deck.
The right side of the cockpit is occupied by the instructor who also has a HUD but is otherwise has stock GII instrumentation. The nosewheel steering has been relocated from the left side to the right, and there is a button that the instructor can press to exit the simulation and return the STA to the standard flight characteristics of the Gulfstream jet.
Externally, to make the Gulfstream fly like the Orbiter, the wings have been modified with three flying surfaces instead of the just two (flaps and aileron) used on the standard GII. The third surface is inboard section of the flaps and is a direct lift flap that can deflect up 30 degrees (this kills some of the wing lift and better simulates the aerodynamics of the Orbiter) and down 20 degrees as a standard flap. The ADAS also moves the flaperon (what was once the flap) and the aileron to faithfully replicate the flight characteristics of the Orbiter. The direct lift flaps are fast acting and also work in concert with the engines. Unlike a standard GII which has clamshell bucket reversers on the engines, the STA has cascade reversers installed that can be used in flight and if they fail, they automatically stow. The wings have also been structurally strengthened and the vortex generators just behind the leading edge are nearly full span to help the airflow stay attached to the wings during the extreme maneuvering performed by the STA. On the stock GII, the vortex generators are only outboard of the wing fence.
Each STA sortie consists of 10 simulated Orbiter approaches, starting from 35,000 feet. The speed is then set at 250 kts (the speed limit of the main landing gear extension) and the main landing gear is extended to create more drag for the steep approach of approximately 20-30 degrees. The ADAS also activates the thrust reversers as needed to maintain the fidelity of the simulation. The descent is flown at 300 kts at 20 degrees which translates to approximately 12,000 feet per minute descent rate. At this point, the pilots are literally hanging forward in their harnesses and only the ground fills the cockpit view. At final approach, the STA is at 250 kts and the instructor lowers the nose gear just in case the trainee pilot inadvertantly lands the STA. In the Orbiter, the point of touchdown corresponds with the STA still about 20 feet off the ground. At that point touchdown is considered to have been made the instructor exits the simulation, takes control and takes the STA back up to altitude for another simulation run.
Most STA flights take place at White Sands, New Mexico, with the aircraft based at El Paso International Airport. Three runways are marked out in the dry lakebed to simulate the runways at Edwards AFB, Kennedy Space Center, and the trans-oceanic abort landing sites at Istres AB, France, Zaragoza AB and Moron AB, both in Spain. Trainee pilots start off with a foundation of 20 STA flights and at this point become competent enough to assigned to an Orbiter crew. Once assigned to a crew, both the pilot and mission commander will fly the STA once a month. Nine months out from launch the STA flights are made every other week and then three months out the STA flights are made weekly. Previous pilots and mission commanders who have already flown to space start their every other week ramp up in the STA at six months out from launch. At three months from launch, the some of the weekly STA flights are made at Edwards AFB and at the Kennedy Space Center as well. Extra flights can also be requested by the trainee pilots.
Two weeks before launch two STAs are flown to the Kennedy Space Center and daily STA training flights are made, one of which is done in the full spacesuit for added realism. By the time a first time Shuttle mission commander blasts off, they will have made approximately 1,000 practice approaches in the STA. First time Shuttle pilots will have made a minimum of 500 STA approaches. With the Shuttle program winding down, no decisions have yet been made on the future of the STA aircraft.
Source: Air International, July 2010, Vol. 79, No.1. "NASA's Unique Approach- Space Shuttle Landing" by Dino Carrara, p82-91.