Trail at End of B-2 Leads to Northrop Contract : Aircraft: Air Force will pay $63.5 million to get rid of the wispy white contrails that give away stealth bomber’s location.


The Air Force has decided to make the Northrop B-2 stealth bomber more stealthy by attempting to eradicate the pesky white contrails the aircraft sometimes leaves in its wake.

The service has awarded Northrop Corp. a $63.5-million contract to develop a top-secret system designed to allow pilots to avoid creating the contrails, which are formed by condensation.

In certain atmospheric conditions, the wispy white lines can linger for up to an hour, leaving a visual track stretching hundreds of miles, according to experts. The problem has plagued military pilots for decades.


“It is not very stealthy when you are flying at the end of a white line in the sky,” said Steven Crow, an aerospace engineering professor at the University of Arizona and a leading expert on contrails.

Air Force officials would say little about the contrail effort, though they were quick to emphasize that it does not reflect any new glitch or deficiency in the $44.4-billion project to build 20 B-2 bombers.

“It is not a shortcoming,” Air Force spokesman Lt. Col. Patrick Mullaney said. “This is not a problem-fixing issue.”

Since Northrop began the B-2 program in 1981, critics have questioned the ability to fly undetected by enemy radar and infrared sensors. They have contended that various types of radar can detect and track all stealth aircraft.

The Pentagon, however, has repeatedly asserted that the B-2 has performed well in tests and that there is no known radar or infrared technology that can pinpoint its location.


The lesser-known issue of contrails seems to be another matter.

Since the early stages of the program, the House Armed Services Committee has expressed concern in secret meetings that contrails would reveal the B-2 to enemy jet fighters or missile crews, according to a former staff member of the panel.


Air Force officials repeatedly assured the committee that the problem was “being worked” on and that a solution would be found, the ex-staff member said.

A committee spokesman said recently that the panel is not even aware of the Northrop contract, even though the award was several weeks ago. “It is surprising that they are just now getting around to doing something about it,” he said.

The long-awaited solution is apparently at hand. Mullaney said the contract reflects recent technological advances, but he said he could not discuss any of the details.

Indeed, methods to control contrails are a closely held military secret, according to aerospace industry officials and combat aircraft test pilots. Academic experts say contrails form in two ways:

* The aircraft’s jet engines burn massive amounts of fuel, emitting water vapor out the tailpipe. As the warm vapor hits the cold air at high altitudes, it condenses. The effect is much like seeing your breath on a cold morning.

* As an aircraft moves through the air, its wings create two intense swirling masses of air, called wake vortices. At the center of each vortex, the temperature and pressure drop compared to the surrounding air, causing humidity to condense and form a contrail. Such contrails are most likely to occur in cold and humid conditions.


Crow said most contrails are formed by wake vortices. But Alvar Bendiksen, an aerospace engineer at UCLA, said engine exhaust is the major culprit.

Whatever the cause, pilots are alert for contrails when outside air is roughly 67 degrees below zero on humid days, according to a veteran test pilot. Contrails are most likely to form at altitudes between 28,000 and 45,000 feet.


If pilots notice contrails forming--which they refer to as marking--they typically lower or raise their altitude. Some planes are particularly notorious markers, among them the Lockheed F-117A stealth fighter jet, the veteran test pilot noted.

In its announcement of the contract, the Air Force said it will cover the development and testing of “hardware, software and air crew procedures that will allow operational B-2 aircraft to avoid or minimize the impact of flying in atmospheric conditions which may produce contrails.”

Mullaney, the Air Force spokesman, said the system will be ready for the last two B-2 bombers off the production line in Palmdale in 1997. The other 18 bombers will be modified later, he said. The design work is being done at the B-2 plant in Pico Rivera.