Why are assumed temperatures decreasing?

Why are the Temperature Assumptions Lower in 2018 than 2007?

A recent article from San Jose Inside suggests that San Jose should prepare for warmer temperatures. This advice is consistent with the City of San Jose’s Climate Smart San Jose “plan to reduce air pollution, save water, and create a stronger and healthier community.”

Why then did the consultant that was hired by the Airport to perform the 2018 One Engine Inoperative study use temperatures (81.3° F) that were almost 7 degrees cooler as compared to what was assumed in the 2007 study (88°)?

2018 Temperature Assumptions are 81.3 degrees F.

2007 Study Temperature Assumptions were 88 degrees F.

This is important, as the higher the temperatures, the more weight (in the form of passengers or cargo) that has to be removed from an airplane to ensure safe operation in the event of a loss of an engine. The change in temperature was the major assumption difference between the 2007 study and the 2018 study.

By using lower temperatures, the economic impact to the airport is much lower than it would be with the assumption of higher temperatures.  And the impact could mean the difference between serving transcontinental/transoceanic flights versus regional destinations, as indicated on SJC’s website:

“Airlines will not fly routes that are not economically practical due to OEI-required weight penalties, and SJC would therefore risk losing existing or potential future air service, particularly to long-haul destinations. This could eventually result in SJC becoming a ‘regional’ airport primarily providing direct flights only to cities along the West Coast and in the western half of the United States. SJC would no longer be able to serve nonstop flights to the East Coast, Hawaii, or overseas to Asia or Europe.” [PDF]

Speaking at the January 28th, 2019 Community Economic Development meeting (YouTube), the Airport’s consultant to the study suggested that he had been conservative in 2007.

“I was typically using 95% reliability for some of the studies back in that 2007 timeframe and invariably I got responses that, that was too conservative and too high. The reason I was using 95% reliability when most of the airlines were using 85% reliability is that if it was a day time operation, the percentages for a 24-hour period, so if the airline is operating mainly passenger flights, not cargo during daylight hours, it would tend to be a little more conservative to use 95%. But, I have really switched to using what the airlines use which is 85% surface temperatures and in-route winds for these type of route analyses.”

This raises several questions:

  1. Who was telling him he was being conservative?
  2. Does each airline use the 85% temperature and reliability numbers? Do some airlines use 90% or 95%?
  3. What about the impact of climate change regarding future temperature assumptions?

Temperatures are expected to rise, so why weren't more conservative temperatures used?





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