Shuttle Training Aircraft

Flying the Ultimate Sim

March 1, 1997

The Gulfstream II in the shadow of the shuttle

Twenty years ago a primer-green Grumman Gulfstream II business jet left the factory in Savannah, Georgia, and flew to the Grumman plant at Bethpage, on New York's Long Island, to be transformed into an important tool for our nation's space program. This aircraft and three others that would follow were destined to fly missions very different from the glamorous task of chauffeuring captains of industry to business or pleasure centers around the world. This particular Gulfstream II would become what many astronauts considered the single best training tool for learning to land the space shuttle as it returns to earth from orbit. Gliding more like a rock than a glider, with no chance to go around, the shuttle crew has only one opportunity to make the perfect landing. Several hundred hours and approximately 900 landings in NASA's Shuttle Training Aircraft — the Gulfstream IIs modified into unique flying simulators — ensure that each crew is prepared to make good on that one chance.

To the uninitiated, the Shuttle Training Aircraft (STA) looks like any other Gulfstream II, although a true aircraft aficionado can readily see that the NASA livery, a plain blue stripe along the fuselage with the NASA logo on the tail, looks rather dull compared to the elegant designs on G-IIs that grace the ramps at flight departments of major corporations throughout the world. Another noticeable difference is the flaps. They deflect upward to spill lift, as well as downward to increase lift. This deflection is needed for a relatively high-lift aircraft like the G-II to successfully simulate the high wing loading of the shuttle.

In 1976 there was concern within the engineering ranks of NASA regarding whether an aircraft designed as a small transport could perform the quasi-aerobatic maneuvers required to fully train a pilot to land the space shuttle. Dives of up to 30 degrees in full reverse thrust would have to be performed repeatedly, as many as 10 times an hour. A normal approach would pull 1.8 Gs. Two-G pullouts would often be required when glideslope corrections were introduced. Some within NASA believed that only a sturdy fighter or attack airframe could hold up under the repeated stress hour after hour. The Grumman A-6 Intruder was closely considered before settling on an executive transport airframe.

The Gulfstream II has proven equal to the task in all respects. The original aircraft is still going strong. The number one STA and the other three have trained every space shuttle crew. Astronauts make each shuttle landing look routine, almost as if they had made the same landing hundreds of times before — something the STA has allowed them to do. Astronauts are quick to credit the training they receive in the STA for their landing success. One unique aspect of this training is that every shuttle commander has logged almost 900 landings in the STA and yet never touched the ground!

The interior of the STA differs significantly from a stock Gulfstream II. For starters, the left side of the standard G-II cockpit was completely moved to the right side. Typically, the aircraft is flown from the right seat and the controls on the left side are completely inoperative. The left side has been outfitted with a full set of space shuttle controls: a hand controller for pitch and roll rate commands; rudder pedals to command a sideslip angle; and shuttle instruments — including attitude indicators and tape meters for airspeed, altitude, and rate of descent. Both sides of the cockpit are equipped with head up display (HUD) devices. A large computer in the main cabin is the heart and soul of the STA.

I dropped by the STA flight manager's office at Houston's Ellington Field to learn what improvements have been made to the STA since I trained for my missions in the aircraft. Charlie Hayes, manager and chief instructor pilot of the STA branch, was eager to tell me about numerous changes that have been made to improve the quality of the course and ensure that astronaut trainees receive the very best training possible. He is proud of the role the STA plays in preparing shuttle commanders and pilots for that one-shot landing at the end of each mission.

Hayes talked through the typical scenario for a training mission at Northrup Strip on White Sands Missile Range near Las Cruces, New Mexico. Most training sessions start at El Paso International Airport with a crew of three aboard the STA: the instructor pilot (IP), the flight simulation engineer (FSE), and the astronaut trainee. The STA departs El Paso with 16,000 pounds of fuel, enough for the 1.8-hour mission. The aircraft climbs in the VFR corridor going north out of El Paso, skirting the White Sands restricted area until clearance to enter is received.

Once the aircraft enters the restricted area, a climb is commenced to a high key representing the 180-degree position of the shuttle from the landing spot. The altitude at this point is about 35,000 feet. During this climb the FSE prepares the computers and avionics for simulation mode operation. The astronaut trainee installs window masking to cover much of the window area, ensuring that the view out of the STA windows is the same as he would have from the shuttle. The masking is never installed until the STA is within the restricted area and under positive radar coverage.

When the STA reaches 35,000 feet and is abeam the point of intended landing, the IP drops the Grumman main gear, which acts as a speed brake. He then selects reverse thrust on both Rolls-Royce Spey engines. In the in-flight reverse mode the Speys are limited to an N2 rpm of 92 percent, although excursions up to 95 percent are allowed. Next, the IP selects the simulation mode, which activates the astronaut trainee's stick. The IP taps the astronaut on the knee to signify that the simulation is beginning.

The astronaut then glides the STA around the heading alignment circle (HAC), following the guidance commands in the HUD or on the HSI guidance bars while cross-checking the view out the windows. Normal guidance has the STA at 20,000 feet and 280 knots equivalent airspeed at a range of 15 miles from touchdown. As the aircraft rolls out on high final it is at 12,000 feet and 7 miles from touchdown. At this point a slight pushover is executed to pick up a 20-degree dive angle and a stabilized 300 KIAS.

At 1,750 feet above the ground the astronaut commands a 1.8-G pullout to intercept the 3-degree inner glideslope. The speed brakes, which have been out at 50 percent if the energy is nominal, are retracted. The shuttle gear is simulated down at 300 feet agl. At this point, the IP puts the STA nose gear down for safety. The astronaut continues to fly the STA flaring just as he would do in the shuttle. A green light illuminates on the panel, indicating shuttle touchdown. If the landing is on airspeed, the green light indicating touchdown occurs when the astronaut's eyes are 32 feet above the runway and the STA's main gear is about 22 feet above the runway. The IP then deselects the simulation mode and manually executes a waveoff, commanding the reversers to stow and spooling up the Speys to climb back for another run.

During the climb the touchdown parameters are called up on the computer and reviewed by the astronaut trainee. The trainee can review his touchdown velocity, sink rate, and distance from the desired touchdown spot. During postflight it is possible to look at traces of all control inputs to detect any tendency to overcontrol or begin pilot-induced oscillations (PIO). During the climb-out, preparations are made for a second simulation run, usually from an intermediate altitude, intersecting the HAC closer to the 90-degree position. On the second and all subsequent approaches, the gloves are taken off and the mission becomes the hellish training session needed to sharpen the skills of the astronaut trainee. High energy, low energy, instrument failures, and navigation distractions are inserted or set up by the instructor to permit the astronaut to see all possible problems that might confront him in the real orbiter. During these abnormal approaches, the 30-degree dive and 92-percent N2 speed limits are often approached. The STA crews are thrown forward against their shoulder straps during the dive to each landing, not a very common occurrence in most business jets.

The STA successfully simulates the subsonic portion of the shuttle's glide to landing. It matches the shuttle's attitude, angle of attack, airspeed, roll and pitch response, and that important eye height at touchdown. The Sperry digital computer, coupled with faster-acting hydraulic actuators and the bidirectional flaps, makes this simulation possible.

The STA provides a model-following simulation. To the pilot it simply means that his input on the rate command stick is analyzed by the computer and interpreted as to what pitch or roll rate of the shuttle is being requested. The computer then commands the proper control surface on the STA to move, producing the desired shuttle motion. This is all transparent to the astronaut trainee as he flies the simulation. Pulling back on the stick produces a pitchup and pushing forward produces a pushover. A response exactly like that of the shuttle is experienced.

In 1977, as the STA was being flight-tested in preparation for government acceptance for training, it was apparent that there were two major problems. First, the lag was excessive in the control system as the computer looked up the shuttle parameters and determined which STA surfaces should be moved. Second, the in-flight thrust reversing produced severe buffeting on the vertical tail, causing the aircraft to vibrate excessively. For a while it looked as though those who felt that a true attack aircraft was needed were right.

After numerous modifications to the reverser cascades, the tail impingement was reduced enough to produce acceptable vibration levels in the cockpit. Then early in 1978, the unenviable task of briefing NASA top management about the remaining problems with the STA fell on my shoulders. I wasn't relishing the prospect of briefing managers at the Johnson Space Center about the severe STA control lag that hundreds of NASA, Grumman, and Sperry engineers were unable to solve. Dr. Christopher Kraft, the famous lead flight director for the early Mercury missions and quite an accomplished control system expert, listened intently as I briefed the control problem. Then he simply said, "Why don't you try a feed-forward signal to get things moving in the desired direction — a signal that could be washed out as the desired pitch and roll rates are achieved." My assembled engineers looked at each other as if to say, "Gee, why didn't I think of that?"

Kraft's unique solution of a feed-forward loop ensured that something started moving as soon as it was requested by stick position. That initial command was washed out as the control logic caught up, but it allowed something to happen immediately in the desired direction of movement, preventing the PIO often found in control systems with too much delay. A few days later we were flying STA test flights without a trace of unfavorable PIOs. That was one of the very few times in my career that a briefing to top management actually resulted in a workable solution.

The STA is often seen on television prior to a shuttle launch or landing, as it is used to check weather conditions on the landing strip at Kennedy Space Center. Special differential GPS receivers, coupled with the aircraft's inertial navigation system, allow the precise measurement of wind shear along the complete subsonic approach path. The STA weather pilot can call a launch hold if at any time he believes that the weather in the landing pattern would prevent a successful return-to-launch-site abort of the shuttle.

Even with the large computer occupying a portion of the main cabin, the STA still has room for six to eight passengers. On January 28, 1986, one of the saddest days in the history of NASA, the STA that had been used to train the Challenger astronaut crew was pressed into service to transport dependents of the deceased crew members back to Houston just a few hours after the fatal explosion.

The STA program has benefited the entire G-II fleet. The aircraft are monitored extensively and have shown that they can take the severe punishment inflicted by training operations. Their high number of pressurization cycles leads the G-II fleet. By all indications, the G-II is a very strong airframe and should easily exceed the present flight time and cycle restrictions.

The perfect landing record of the space shuttle program is silent testimony to the excellence of the training that astronauts receive in the Shuttle Training Aircraft. These unique aircraft will be around as long as the space shuttle is operating — which, by the look of things, could well be into the next century.

Bob Overmyer, a former space shuttle astronaut and AOPA Pilot columnist, completed this article a few weeks before his death in an airplane accident in March 1996.