The pilot of the fifth helicopter scheduled to lift off the deck of the Nimitz aircraft carrier was following the special procedures for a top secret mission.

He revved up his twin-jet engines to full power, studied one instrument after another in the cockpit of the vibrating Navy RH53 Sea Stallion and held his position on the deck longer than usual.

There could be no malfunctions on this one. His chopper was one of eight that were supposed to make history by snatching 53 American hostages out of the grip of Iranian militants in Tehran.

The anonymous pilot of Chopper Five, whose identity the Pentagon has not yet disclosed, played a crucial part in the drama of bad luck which led to the scrubbed mission's failure. If Chopper Five had made it, the history would have been written differently.

The officer in the tower high above the canted deck of the giant Nimitz radioed into the pilot's earphones: Cleared for takeoff. His instruments said: Everything is go.

With silent thanks to the two separate crews that had gone over Chopper Five inch-by-inch to make sure it was ready for this long flight, the pilot used his hands and feet together with the skill of an organist and slipped his bird off the deck of the Nimitz.

After clearing the deck, Chopper Five ran low over the waves, nose pointing down for a bit to gain speed, then swung onto its course for Iran. The pilot had been briefed extensively on this mission. He knew it would be exhausting, difficult and dangerous.

For two hours Chopper Five thud-thudded first over the dark Arabian Sea and then over the hilly terrain of southern Iran. Nothing unusual to report. Darkness obscured the land features so vital to this chopper, which flew low over the desert to duck under the beams of any searching Iranian radar. But the pilot and copilot had instruments for flying blind.

The "wet" compass swings wildly around in the cockpit if the chopper is forced into violent maneuvers. But another compass, steadied by a gyroscope, would show the pilots where they were going without flinching, no matter what the helicopter did.

The gyro, spinning around to provide this stability, needed a little electricity to operate. The power came from one of those black boxes bolted onto the frame of the big helicopter.

Another instrument that made blind-flying possible was the attitude indicator. It is a little instrument that show the pilot the position of the helicopter in relation to the horizon -- whether he was tipped sideways, right side up or upside down.

Chopper Five also carried those old fashioned instruments that go all the way back to Eddie Rickenbacker, an altimeter showing how high off the ground the helicopter is, a needle-and-ball to show the pilot how sharp he is turning and backing. There was also instruments showing forward and downward speed.

The trouble with these old-fashioned instruments is they take a while to react. This is no big deal in a clear sky thousands of feet above the earth but delay can be fatal if the chopper is flying blind a few hundred feet off the earth at 150 miles an hour or more. The gyro compass and attitude indicator are, thus, vital for blind flying at low altitude.

What the pilot and crew of Chopper Five did not realize as they were flying along that starry night of April 24, flying toward a rendezvous with seven other helicopters and six C130 transport planes in a stretch of desert near Tabas, was that someone on board had left his jacket jammed against an air vent. That particular vent fed cool air onto the black box sending electricity to the gyro compass and attitude indicator.

Over southern Iran, Chopper Five ran into the edge of what was to be a violent sandstorm -- a bath of sand the texture of talcum powder "Like flying in a milk bowl," as the pilot would say later. And the black box feeding electricity into those vital navigational instruments was getting hot -- for want of cool air from the blocked vent.

The pilot and copilot strained to keep their chopper straight and level, resisting the temptation to trust their sensory judgment of which way was up and which way was down. The sandstorm buffeted the ungainly craft, caught at the moment in a valley ringed by sharply rising hills. An accident waiting to happen. The crew had been trained to trust their instruments, not themselves, as they worked the controls with deft touches of both their hands and feet.

Behind the pilots, the air crewmen were being tossed around. They were getting air sick. The pilots were becoming disoriented from the buffeting, at the edge of the condition called vertigo.

At this critical moment, when the pilots were barely winning their fight against the desert storm, the gyro compass went out completely. One of the two attitude indicators, one for each pilot, went out, too. The black box burned itself out.

The pilot decided to keep plunging ahead into the thicker part of the dust storm to make his rendezvous and risked ramming into a mountain he knew to be near. Then he decided to reverse course to reach the clear air where he could read the ground, where his old fashioned instruments could guide him back toward the Nimitz.

The pilot could not know at the time that another helicopter had already been abandoned on the way to Desert One. A second would not be able to take off because of hydraulic failure. As Chopper Five swung back, the Nimitz was steaming toward it, trying for a rendezvous in the Gulf of Oman before the helicopter's fuel ran out.

Adm. Thomas B. Hayward, chief of naval operations, was to say afterward that when the crew of Chopper Five turned back to the Nimitz, the mission was finished: "With them went the prospect of mustering the needed six operational helos. . . . Therein lies the real story of this aborted mission."