Complaints about jet noise from residents near O’Hare Airport dropped 90 percent from January to February — but the skies over the western suburbs and Northwest Side are as busy as ever.

An anti-O’Hare noise group says that the drastic drop in just one month was caused by change made by city officials to block an app the group created.

In January, 252,878 complaints about jet noise were logged with Chicago officials. In February, the city recorded only 33,499 — the lowest number since December 2014, according to data released Friday by the O’Hare Noise Compatibility Commission.

In May, complaints reached an all-time high of 606,591, according to the commission’s data.

“The city is actively distorting the 311 system,” said John Kane, the chairman of the Fair Allocation in Runways Coalition, which operates chicagonoisecomplaint.com, which is designed to make it possible to object to the racket with one click by storing complainants’ personal information.

Those who submit a complaint online via the city’s 311 system must now click a box to certify that they are not a robot through the CAPTCHA program that is designed to prevent automated systems from submitting data online.

A recent “spike” in the number of automated noise complaints 15 times higher than the usual rate submitted online to 311 threatened the citywide system, said Lauren Huffman, a spokeswoman for the Chicago Department of Aviation.

“In response, CDA adopted the new CAPTCHA feature in January as part of a citywide effort to enhance customer service for all residents who contact 311 and rely on its services,” Huffman said in a statement. “The adoption of this feature will also improve the integrity of the noise reporting system online and the ongoing tracking of this issue.”

Kane said his group’s app was not designed to circumvent the city’s complaint system but to give residents a way to complain about each instance of jet noise disturbance. Some homes have jets passing by as often as a dozen times every hour, Kane said.

“The fact of the matter is that 33,000 complaints does not accurately reflect life under the flight path,” Kane said. “The city obviously does not care.”

Approximately one-third of all the complaints logged in February came from Chicago, with the majority of those complaints coming from the 41st Ward, which includes Norwood Park and Edison Park, according to the data.

Most suburban complaints came from Elk Grove Village, according to the data.

Complaints can also be made by calling a 24-hour hotline — 800-435-9569.

In 2013 and 2015, two east-west runways opened as part of the $8.7 billion O’Hare Modernization Program, sending hundreds of flights over areas of the Northwest Side like North Park, Jefferson Park, Edgebrook, Edison Park and Norwood Park that previously heard little or no jet noise.

A sixth and final east-west runway is set to open in 2020.