Early in the crash investigation of EgyptAir Flight 990, the airline said the crew had passed a psychological examination. But in truth, no pilot gets a formal psychological or psychiatric examination unless he or she raises suspicions during routine medical exams, acts strangely around colleagues or voluntarily reports problems.

A pilot who is good at masking internal problems often can get away with lying about mental conditions, according to government officials and physicians who did not want to be identified because of the sensitive international implications of the EgyptAir crash.

Nonetheless, instances of suicide or severe self-destructive behavior among commercial pilots are so rare that suicide would hardly be more than an asterisk in a list of causes of plane crashes. More often, suicide or suspected suicide occurs among general aviation pilots who use an aircraft to kill only themselves.

No one has revealed the details of co-pilot Gameel Batouti's last medical exam or any other medical or psychological records, but on the surface there is no indication that he would ever have been inclined to push an airliner full of people into a fatal dive. That history is not always an accurate predictor of behavior, however.

"I have seen a number of pilots develop problems after years of flying," said a senior federal official. "It can happen any time in a career."

Aviation has wrestled with the question of suicide and pilot mental problems for decades. In a 1971 report to the Federal Aviation Administration, a research team led by Robert E. Yanowitch suggested that some accident investigations include a detailed "psychological autopsy" to determine why a pilot took actions that seemed self-destructive.

The National Transportation Safety Board does detailed human-factors investigations of pilots, particularly of the last 48 to 72 hours before the crash. But the psychological autopsy suggested by Yanowitch would be far more intrusive.

Yanowitch acknowledged the difficulties in conducting such an intrusive examination and noted that families sometimes go into denial and even destroy evidence such as suicide notes.

"It may be necessary to explain to informants that self-destructive tendencies do develop in responsible, religious or successful persons, and the fact that the deceased may have made plans for the next day or week is not sufficient reason to rule out a simultaneous preoccupation with suicide or a suicide plan," the report said.

The report also noted that it is not necessary for a person to be planning suicide to exhibit self-destructive behavior. Destructive impulses "ordinarily are well-controlled and most unconscious," the report said. "However, these impulses can be brought to the fore and released under influences of mental stress, physical exhaustion, frustration, alcohol, drugs and other tangible and intangible agents."

Many commercial pilots seem to have several stories of the sudden onset of odd behavior among their colleagues, which on rare occasions became so bad that they reported a fellow pilot to the airline or to the professional standards boards of the Air Line Pilots Association.

A former co-pilot who did not want his airline or the airport identified recounted that while approaching a major airport on a clear evening, a senior captain suddenly leveled the plane at 1,000 feet and proceeded as if he would simply fly over the airport.

"I said, captain, you're at 1,000 feet," the co-pilot said, evoking no reaction from the pilot. The co-pilot repeated the statement more forcefully. Just as the co-pilot decided to take control of the plane, the captain seemed to snap out of his fog. Rather than aborting the landing, the captain went into a precipitous descent and successfully landed.

The incident illustrates one of the main reasons why there are rarely crashes of commercial airliners from aberrant pilot behavior or suicide: There are two pilots in the cockpit.

The screening of pilots begins with physical exams performed by FAA medical examiners. The FAA usually uses practicing physicians, about half of whom are pilots, who perform the low-fee task because they like aviation. Only a small number are psychiatrists.

The standard FAA medical form contains a number of psychological questions designed to spot neuroses, seizure disorders, neurological problems, substance dependence (other than tobacco and caffeine) and other psychological or medical conditions that could affect a pilot's mind.

An honest answer to some of these questions could end, or delay, a career.

For instance, the FAA will not allow a pilot to fly an airliner if he or she is taking an antidepressant, such as Prozac.

But if a pilot lies on the form, there are generally only two ways to discover the lie: The medical examiner's suspicions are raised by the applicant's behavior or the applicant has been convicted of driving under the influence. The FAA routinely checks only the national driver register, not any of the pilot's medical or psychological records.

However, if there is anything on the form that raises suspicions, including adverse comments by the examiner, the FAA computer at Oklahoma City will flag the application for further examination by a team of specialists. Those specialists may then ask for records or order a psychiatric or psychological examination of the pilot.

Captains are required to undergo a more rigorous "first class" exam every six months. Co-pilots usually are required to have a "second class" exam once a year. While 194,664 first-class certificates were issued in 1998, 633 pilots were denied a certificate for "neuropsychiatric" reasons, 118 of those for serious disorders such as clinical depression, anxiety, hypochondria and obsessive-compulsive disorders. A total of 98,684 second-class certificates were issued in 1998, while 626 applications were turned down for neuropsychiatric reasons--151 of those for serious disorders.

Pilots also police themselves. Before hiring, most airlines require pilots to survive tough questioning by a panel of pilots who want to determine if the applicant fits the company's culture. If they suspect some problem, the applicant is turned down. The pilots' union also has professional standards boards, which can investigate reports of aberrant pilot behavior.

Pilots throughout their careers are also trailed by a platoon of trainers, inspectors from both the FAA and the airline, and the pilots they fly with and whose lives depend on their performance.

CAPTION: Since preliminary study of EgyptAir Flight 990's cockpit voice recorder, above, investigators have questioned the intentions of the plane's co-pilot.