Long descents would save fuel and reduce noise but probably won't be implemented for arrivals at Minneapolis-St. Paul International Airport (MSP) because of the hub, according to a SMAAC communication to Minneapolis‚ MSP Noise Oversight Committee (NOC). Minneapolis‚ Representative, Council Member Scott Benson agreed to try to re-open noise-abatement versus noise mitigation discussions at NOC.
"This is an opportunity for NOC to address how higher rates (more operations per hour) increases noise on arrival paths as well as increasing noise exposure on dispersed departure paths," according to SMAAC President Jim Spensley. "Perhaps even more important, lower rates would also be safer." SMAAC referred to a recent study conducted by John-Paul Clarke, an aeronautical engineer at MIT, with colleagues at Boeing and NASA.
Benson said the NOC had previously "asked FAA for approval of long descents at MSP, particularly at night. The airlines were attracted by fuel cost savings to join in the request."
Instead of beginning descents 10 miles away from 4,000 feet, long descents in a controlled test at Louisville International Airport (not a major hub) started some 40 miles away from a height over 11,000 feet, and reduced noise levels by between 3.9 and 6.5 decibels compared with conventional approaches. Pilots easily maintained an angle of descent of 3 degrees and kept engine power changes to a minimum. According to Clarke, the Boeing 767 aircraft used in the tests saved over 450 pounds of fuel per landing as well.
SMAAC notes that several operational and flight-path changes, associated with higher rates (flights per hour) limit noise abatement at MSP. The airport‚s noise compatibility program and noise exposure map for 2007 could be, and should be, compared with current operations by computer modeling, mapping where hub expansion increased noise.
"A lower-rate MSP operations plan with departure paths over less developed areas and long descents for approaching flights optimizes noise abatement." Spensley said. "The current noise exposure map, made from flight-paths during a few low use days in April 2002, purports to represent noise exposure in 2007, but rate changes made since 2002 were not assumed, not modeled, leading to a gross under-representation of the areas where mitigation (sound insulation) conditions apply.." Benson said that he agreed. Take-off procedure changes begun in 2003 were obviously not input to the computer model in 2002, and fewer-than-actual flight paths were modeled, inaccurately computing noise-exposure contours. Benson intends to ask for corrected inputs, including a model optimized for noise abatement as a baseline and a one reflecting actual operations to better define mitigation needs.
"When re-modeling is rejected by NOC, or by MAC, as I predict it will be, perhaps the City Council will get it that participating on NOC is counter-productive." Spensley remarked, "SMAAC has suggested withdrawal from NOC three times since 2003 because models were not refined as conditions, rates, and flight patterns changed. If high arrival rates alone double or quadruple (3 db to 6 db) ground noise compared to what is possible for the same number of flights per day but lower peak rates, Part 150 Rules require a new noise-exposure map." Minneapolis and other cities are suing the MAC over its plans to reduce the Sound Insulation Program. SMAAC says that the 1998 agreement to provide 5 db noise insulation further from the airport (out to 60 ldn) was based on the knowledge that Part 150 noise models were at least that far off as a result of the airlines‚ under- projecting daily use and delaying deployment of newer and less-noisy aircraft.
Background. John-Paul Clarke, an aeronautical engineer at MIT, and colleagues at Boeing and NASA developed a quieter continuous descent path into Louisville International Airport with "... no transients in thrust over noise-sensitive locations." Working with the cargo airline UPS, Clarke had the crews of two Boeing 767 jets use a longer approach while researchers collected comparative noise data at 14 ground locations along the approach path.