The U.S. military yesterday suffered its fourth fatal air crash in three days in the Persian Gulf area, increasing the number of accidental deaths to 31 since Operation Desert Shield began and renewing concerns over the safety of troops in the dangerous desert environment.

Some military authorities blamed the spate of aircaft mishaps on the treacherous desert flying conditions and the heightened intensity of aggressive combat training in a region threatened by enemy forces.

Fourteen crew members have been killed in 14 major helicopter and airplane crashes along the Arabian peninsula, including two in yesterday's downing of an F-111 bomber, since the United States began dispatching troops to the region two months ago. Another 13 military personnel died when a C-5 transport bound for Saudi Arabia crashed in Germany during takeoff last month.

While military authorities say those numbers are not unusually high for an operation in which 200,000 troops are working under such rigorous schedules, the incidents have prompted the Army to equip its helicopters in the region with new safety equipment and to require pilots to undergo extra training before they begin flying missions over the desert.

"That desert is one of the toughest environments in which to fly," said one Marine Corps official familiar with the crashes. "You have to depend on instruments to a large extent because you lose the horizon, everything is flat, sand kicks up and you can't tell the ground from the sky. You may not be

able to see a sand dune in front of you."

Army officials said that all five of their helicopter crashes involved pilots striking unseen sand dunes -- some 100 feet high -- at night. Seven crew members have been injured but none killed in those mishaps, officials said.

A spokesman for the Air Force Tactical Air Command said yesterday the "threat of large-scale combat . . . heightens the intensity of training and necessitates an aggressive flying attitude seldom required during peacetime training."

Four of the five fatal air crashes in the Persian Gulf region have occurred this week. On Monday, two U.S. Marine Corps UH-1N Huey transport helicopters disappeared into the Gulf of Oman with eight men aboard, and an Air National Guard F-4 Phantom fighter jet crashed in the United Arab Emirates on the southeastern edge of the Arabian peninsula, killing both of the officers aboard.

Military officials described the rash of air accidents as "tragic" but said the number of crashes, deaths and injuries is not statistically higher than that recorded during peacetime operations.

Even so, Army officials said the service will begin installing extra safety devices on its aircraft and on crew members' night vision goggles. In addition, the Army has barred pilots from low-level night flights until they complete safety training programs in the gulf region.

All the Army helicopter crashes and most of the other air mishaps have occurred during night training, officials said.

"The majority of our training is at night," said Army spokeswoman Maj. Nancy Burt. "We train as we will fight -- most of the fighting will take place at night."

Four of the five Army helicopters that crashed on the Arabian peninsula were OH-58 scouts. Army authorities said 30 Army helicopters have crashed worldwide during the first nine months of this year, of which at least 10 were OH-58s. Since the beginning of the year, 30 Army personnel have died in helicopter crashes.

The Army has accelerated installation of several new safety devices on helicopters in the gulf region, including radar altimeters that flash when a helicopter is flying too low and an audio device that beeps in the pilot's earphones if the

aircraft drops beneath safe altitudes.

The Army also plans to install "heads-up display" devices on night vision goggles to reflect digital readouts of instrument panels so pilots do not have to shift their eyes so frequently from the panel to the exterior of the helicopter.

Night vision goggles had been criticized as a major cause of nighttime helicopter accidents prior to the Saudi operations by pilots who said they could actually impair vision. Military authorities argue that the goggles, which use starlight and other light sources to enhance night vision, are a great advantage to well-trained pilots.

Although the pilots on some helicopters were using the goggles when they crashed, at least one chopper plunged into a sand dune after the crew had taken off their goggles, an Army official said.

Five Marine helicopters have crashed since the beginning of the August deployment, of which the most serious incidents involved the two UH-1N Hueys in which the eight crewmen were killed. Initial reports indicate the choppers may have crashed into each other over the water, officials said.

The Marine Corps has had 24 aircraft and helicopter crashes throughout the service this year, resulting in 16 fatalities including the loss of the two crews in the Gulf of Oman.

The Air Force has lost three jets and six crew members on the Arabian peninsula, while another 13 people were killed when a C-5 transport en route to Saudi Arabia crashed in Germany shortly after takeoff.

In fiscal year 1990, which ended Sept. 30, the Air Force recorded 50 aircraft crashes and 44 deaths, a spokeswoman said. Last year the service had 50 crashes and 76 deaths.