Weather and Other Noise Factors

Wind, air temperature, humidity, cloud cover, and temperature inversions all affect the transmission of noise and the loudness heard on the ground. These atmospheric factors vary from day to day, and so will the noise level heard on the ground. Wind, for example, moves the air and the noise it transmits. As a result, noise from a particular location may be heard loudly at a home on one day, but not at all on another. On humid days, the air has more density and noise, and particularly, certain noise frequencies will be heard further away from their source. A temperature inversion or cloud layer may reflect noise back to earth.

Federal standards specify the weather conditions under which the Ldn contours are calculated. But weather is not fixed, so unlike altitude contours on maps which define a fixed condition, the noise contours provide a calculated planning guide. As a result, the equivalent of 65 Ldn could be heard under certain weather conditions, even outside the Aircraft Noise Impact Overlay District.

Aircraft Size & "Visual Intrusion"
Small aircraft are obviously less visually intrusive than large ones. At an altitude of 600 ft. a small 19-passenger aircraft may pass almost unnoticed, while a 400-passenger airliner would be very noticeable, even if it was totally silent.

Much of the growth in flight operations at Washington Dulles today is with small, relatively quiet regional jets. However, the long-term trend in aviation is toward larger aircraft. As flight operations grow at Washington Dulles over the years, flights likely will become more, rather than less visually intrusive.

Noise Standards
Federal regulations require that commercial aircraft meet noise standards which are "technically feasible and economically reasonable." As engineering knowledge has advanced, aircraft have been required to become steadily quieter, but we are now at the point where aerodynamic noise is nearly as large a factor as engine noise for some new aircraft. Aerodynamic noise heard on the ground comes from air flowing over the aircraft's undercarriage, flaps and control surfaces during takeoff and landing. While research is underway to try and reduce aerodynamic noise, the simple fact that this "wind" noise is now a factor alongside engine noise means that the engineer's ability to further reduce aircraft noise is meeting the rule of diminishing returns.

What is technically feasible and economically reasonable is a function of the aircraft's purpose. For example, the laws of physics suggest that supersonic airliners are always likely to be noisier than a subsonic aircraft of equivalent carrying capacity. Military aircraft, incidentally, which are regular visitors to Washington Dulles, are designed for a different purpose and do not have to comply with civilian noise regulations.

Flight Operations Vary
At most airports, including Washington Dulles, aircraft do not follow fixed flight paths away from the airport. Once airborne with sufficient altitude, pilots are free to take up the heading that will carry them to their next navigation point with minimum fuel burn and time loss. Very large four-engine aircraft will climb more slowly than small twin-engine aircraft, which may thus turn onto their course sooner. Arriving airliners regardless of size all follow a stabilized three-degree glide slope as they approach the runway.

The above illustrations show aircraft altitude at various distances from the airport on approach for all aircraft and on departure for a four-engined 747-200, assuming a straight out departure. Two- and three-engined aircraft will climb faster.

The picture above represents the radar paths of all aircraft within 10 miles of Dulles on a given day. Please note that many of the aircraft that could be heard and seen within that 10-mile square relate to other airports. Only the blue and red paths relate to aircraft arriving and departing Washington Dulles.

Safety
Even with the horrors of terrorism, air transportation is way and above the safest means of transportation. However, aircraft accidents do occur.

Statistically, nearly 70% of all aircraft accidents occur on takeoff and landing, with the majority being in close proximity to the airport.

Introduction
County Policies
Maps
Survey
Growth of Dulles
WATF Recommendation
Frequently Asked Questions