The Air Affair

Ask Propellerhead

Propellerhead answers your most burning aviation trivia questions, whether silly or serious.

Q. What is the purpose of those strands of purple yarn attached to the wings of some airliners?

A. The McDonnell Douglas (now part of Boeing) MD-80 airliner stores jet fuel in its wings. During long flights at high altitutes, the jet fuel becomes very cold, well below the freezing temperature of water. Then, when the airliner descends into warmer but humid conditions (like Florida in the summer), the super-cooled fuel can cause moisture in the air to form a layer of ice on the wings. This ice layer can dangerously disturb the airflow over the wings. But because the ice layer can be smooth and completely clear, it can be difficult to identify. So, strands of yarn are attached to the wing as ice indicators. If the strands move freely in the wind, or when poked at with a pole by the pilots (Propellerhead kids you not!), then no significant ice is present. If the strands are stuck fast in an ice layer, the aircraft must be de-iced before its next flight.

Q. Since the Earth rotates at a speed of nearly 1,000 miles per hour at the equator, why don't airplane flights going east take much longer than the same flight going west, and how does an eastbound light aircraft flying at a mere 100 knots or so keep up with the rotating Earth at all?

A. Friction with the Earth's surface causes the atmosphere to rotate at about the same speed as the surface. Therefore, the air through which the aircraft is flying is rotating about the Earth's axis right along with the ground below, and the aircraft is carried along with the air.

Q. What happened to the "rocket backpacks" that the U.S. military was developing in the early 1960s?

A. Only a few examples of the "rocket belt" as it was called, were built by Bell Aerosystems before the military lost interest. The main drawbacks were that the device could carry only enough fuel for about a half-minute of flight time, and the fuel itself was hydrogen peroxide, and extremely hazardous chemical in the concentrations needed for the rocket belt. After military research ended, Hollywood used a copy of the device in movies, TV commercials, and special events (such as the 1984 Olympic Games in Los Angeles). But the rocket belt never became anything more than a curiosity.

Q. Who was the guy who flew above Los Angeles in a lawn chair suspended from helium balloons?

A. On July 2nd, 1982, 33-year-old truck driver Larry Walters tied 45 six-foot helium-filled weather balloons to an aluminum lawn chair, and launched himself into the skies above San Pedro, California, a seaside community about 20 miles south of Los Angeles. His northeasterly course carried him over the Long Beach Harbor, then crossed the approach path of Long Beach Municipal Airport. The contraption was reported to air traffic controllers by two airliners on approach to that airport. Walters said he had intended to soar to the Mohave Desert to see an upcoming Space Shuttle landing. But he decided cut the flight short after less than an hour when the rarified air at 16,000 feet left him chilled and lightheaded. Walters missed his intended landing target of a large grassy area in Long Beach, and instead came down tangled in some high-tension power lines. Walters was uninjured, but power was knocked out to a portion of the city when rescuers were forced to cut the lines to get him down.

Unable to suspend the pilot's license that Walters did not have, the Federal Aviation Administration fined him $4,000 for operating an aircraft without an airworthiness certificate, and entering the Long Beach Airport airspace without making two-way radio contact with the control tower. Walters was eventually able to bargain the fine down to $1,500.

Walters was never able to capitalize on his brief fame. He was unsuccessful on the lecture circuit, and drifted into obscurity. On October 6th, 1993, Walters committed suicide in the Angeles National Forest.

Q. I want to build a lighter-than-air craft. How do I estimate how much weight a given volume of helium will lift?

According to The Noble Gases by Isaac Asimov, helium has a weight of 0.178 grams per liter at ordinary atmospheric pressure. Ordinary air is 7.25 times as dense, at 1.29 grams per liter. Each liter of helium will therefore lift 1.112 grams of dead weight.

For a real-world example, a six-foot (approximately 2-meter) weather balloon has a volume of 4.189 cubic meters, or 4189 liters. Filled with helium, the balloon would lift 4658 grams, or about 10.25 pounds. The 45 such balloons used by Larry Walters would have a combined lift capacity of over 450 pounds, which is why he zoomed to such an unexpectedly high altitude. Of course, the actual buoyancy would be slight less, since the balloon must be filled to a pressure slightly higher than ambient in order to maintain its shape.

Q. Since hydrogen weighs half as much as helium, would I get twice as much lift from hydrogen-filled balloons?

A. No. Even though a given volume of hydrogen will indeed weigh half as much as the same volume of helium, both gases are so much lighter than ordinary air that hydrogen will provide only about 6 percent more lift than helium. The high risk of conflagration is not worth such a minor increase. In fact, even a theoretically perfect buoyancy device, a shell enclosing a total vacuum, would lift only about 15 percent more than helium.

Q. What became of the ducted-fan-powered "aircar" that appeared on the covers of science magazines and in future-predicting TV shows?

A. Paul Moller and his company Moller International in Davis, California (north of Sacramento) continue to work on his futuristic vehicle. Moller was most recently in the news for his work on small autonomous flying vehicles designed for the military for aerial reconnaissance. The hovercraft-like vehicles use the rotary engine and guidance systems he continues to develop for his aircar. For more information, see the Moller web site at

Q. What is the name of the book that tells the story of the Air Canada airliner that ran out of fuel and glided to a landing on an unused airstip?

A. FREEFALL: A True Story by William and Marilyn Hoffer, St. Martin's Press NY, 1989, ISBN 0-312-02919-5.

Q. Tell me about the "aircraft graveyard" run by the U.S. military somewhere in the southwestern U.S.

A. The official designation is The Aerospace Maintenance and Regeneration Center (AMARC), located at Davis-Monthan Air Force Base in Tucson, Arizona. Nearly 3,700 aircraft are stored on the 2,700-acre facility. All excess military aircraft are stored at AMARC. Aircraft don't come here only to be scrapped. Many will be sold to foreign governments, transferred to other government agencies, such as the Forest Service for water bombers, or used for spare parts. Tours of the expansive facility are offered to the public by the Pima Air Museum. For more information, visit the museum's tour information page.

Q. What is the formula to compute the distance over the surface of the Earth between two points, from their lattitude and longitude?

A. This is known as a Great Circle distance. Dutton's Navigation and Piloting, 14th Edition, by Elbert S. Maloney (1985, United States Naval Institute, ISBN 0-87021-157-9), presents the formula to compute Great Circle distances. The values used in the formula are:

Lat1, Lon1: latitude and longitude of origin, in degrees
Lat2, Lon2: latitude and longitude of destination, in degrees

The actual formula is:

D = 60 * arccos[ sin(Lat1)*sin(Lat2) + cos(Lat1)*cos(Lat2)*cos(abs(Lon1-Lon2))]

The resulting value of D is the Great Circle distance in nautical miles.

If the origin and destination are on different sides of the equator, insert Lat2 as a negative value in the calculation.