Dateline: 22 July 2003

South Metro Airport Action Council ~SMAAC~
PO Box 19036, Minneapolis, MN 55419

July 22, 2003

Ms. Vicki Grunseth, Chair
Metropolitan Airports Commission
6040 - 28th Avenue South
Minneapolis, MN 55450

Regarding the Part 150 Noise Control Program Update.

Dear Chair Grunseth:

We are sending this letter directly because Mr. Leqve responded to our letter delivered at the Fall session, and indicated that few of our comments would be forwarded to the Noise Oversight Committee or the Commissioners. Our certified letter of May 22, suggesting that more formal hearings be conducted before the Commissioners, was received at MAC General Offices, but it was not acknowledged by the Commission or recognized when I asked if it had been considered at the Commission's June meeting.

At the quarterly noise meeting held by MAC staff today, SMAAC made the following comments:

1. Fleet-mix assumptions.

A. In assuming the retirement of some noisier planes and assuming the remaining noisier planes will not fly more often, the Commission is projecting reduced noise exposure (contours) by 2007. This does not properly account for noise exposure between 1996 and 2007, just as delayed retirements did not between 1992 and 1996. Will the updated Noise Control Plan say what will be done if the 2004 to 2007 fleet-mix and use differs a lot from the assumptions

B. Will MAC correct noise exposure maps to actual 1996 to 2006 conditions and provide relief in areas exposed to non-complying noise, comparable to DNL-projected 70+DNL and 69-65 DNL noise, but not yet treated?

C. Will MAC consider the fact that past noisy-aircraft retirement projections were inaccurate, and that partial retirements in a period of more MSP flights per day per aircraft added considerable noise compared to projections? If MAC is planning to (use increased capacity to) attract competing airlines, then assumptions about the fleets of candidate airlines reasonably should be considered. Were these data collected, and are the fleets of the likely new users noisier? D. MAC regards the fleet-mix assumptions confidential and an audit is impossible without access to the base data. What differences in projected airline use would trigger a revision of the noise exposure map? [Note: NOC brought this issue up, but took no action. The FAR suggests a revision if changes would apparently increase or decrease mitigation eligibility or if a 1.5 dbA peak noise intensity difference is observed.]

2. Flights per day: HNTB vaguely answered NOC Representative Scott Benson's question about INM sensitivity to total flights by saying that since average noise is a log function, INM noise contours increase more slowly than total flights as airport use is increased. SMAAC notes that projection of annual flights is more of an operational issue than a noise issue, and fleet-mix and time-of-day projections are much more important in INM computations. However, in deriving these and other INM inputs, HNTB has made several operational assumptions (generally related to peak-hour use) which have not yet been considered by FAA, have not been made public, and may not be safe and feasible. 3. Time of day: The INM Technical Manual refers to the 10 db night penalty as a correction for atmospheric noise conduction, applied because mean annual temperature is used to compute noise attenuation in the INM. That is, actual noise conduction is higher when temperatures are lower, and therefore noise intensity on the ground at night is higher for the same aircraft at the same altitude. It is possible to set up the INM to use a variety of temperatures to more accurately model airport conditions. Since abatement is preferable, the 10 db penalty hopes to limit night flights. Since the "penalty" results in increased mitigation cost to FAA and MAC, not the airlines, has MAC considered increasing fees for, or placing restrictions on, night flights to reduce mitigation cost exposure?
MAC staff has publically claimed that the 10 db penalty is applied to flights more likely to interfere with sleep, that is, only flights between 10 PM and 6 AM. In fact, so few flights are needed at these hours that little or no effect on the noise exposure map could be expected.

Is the 10 db penalty to be applied in the model to flights from 6 PM to 6 AM? If not, how many projected flights per 24-hour day are assessed this penalty? How many flights are projected between 6 PM and 10 PM? Considering current use of MSP, how well do 2007 passenger projections reflect percentages of flights in these three time periods? Staff suggested that these comments are applicable to "INM input files." SMAAC notes, however, that no opportunity has so far been scheduled for hearings or public comments on "INM input files" before HNTB recommendations are completed, or, on the current schedule, before approval. Moreover, the INM models a daily schedule to compute noise exposure, use of MSP varies with demand, and demand with time-of-day. Simply dividing the number of annual flights now being projected from passenger demand would misrepresent daily use. Therefore, either HNTB already derived projected annual flight levels, perhaps by varying load-factors by time-of-day, considering how passenger use is distributed over a typical day [which seems to be the case]; or, MAC intends to revise the annual flight assumption after considering daily operational rates, runway use, and schedules.


James R. Spensley,
President, SMAAC

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