Denver International Airport collects more than $570 million annually in airline revenue but auditors found that billing errors have resulted in one airline being overcharged by nearly $6 million, according to a new report released Thursday by Denver City Auditor Timothy O’Brien.

Specifically, audit reports show that DIA overbilled United Airlines, one of its largest tenants, by $5.94 million, largely because the airport charged the airline for using gates it was already paying for.

The Denver Gazette reached out to United Airlines for comment but the airline has yet to respond.

Airports invoice airlines for various services, including the use of their terminal gates and associated support infrastructure, such as jet bridges.

Additionally, auditors identified failures to enforce contract payment terms that cost the airport itself $1.6 million in interest penalties for late payments.

Auditors also found that while contracts permit the airport to charge 18% on late airline payments, DIA, in most cases, did not enforce those penalties, leaving $1.6 million in interest-penalty revenue that could have been collected from United Airlines and Frontier Airlines between 2022 and 2024.

“The airport overly relies on a year-end settlement process to correct billing errors not discovered during the year,” O’Brien spokesperson Michael Brannen said in a press statement. “But the year-end settlement process may not catch all errors from staff, like the $352,000 in credits we found the airport missed and owed to United for 2024.”

Auditors also found that Frontier and United had more than $13.7 million in outstanding and unapplied credits as of December 2025.

Large outstanding credits complicate the airport’s ability to charge interest against overdue payments from airlines, Brannen said.

The airport disagreed with the auditors’ recommendation to reduce large, lingering credits, citing a restriction from use and lease agreements that revenue-sharing credits can only be applied against future rates and charges.

Auditors note they found the same issue during a similar airline agreement audit in 2016.

“Specifically, we found that DIA lacks effective practices for ensuring timely payments from airlines, prompt collection of delinquent airline payments and associated interest penalties, managing airline space changes, managing agreements and revenue collection, and succession planning for DIA finance personnel,” the 2016 audit stated. “These weaknesses negatively impact DIA’s ability to properly manage airline agreements.”

However, auditors point out that, as a positive step, compared to the 2016 audit, airport staff are billing airlines more quickly when the airlines change the spaces they lease.

DIA representatives agreed to seven of the eight recommendations from auditors.

“Denver International Airport has proven it can adjust to more efficient processes,” O’Brien said. “We will conduct a follow-up audit to see whether the airport addresses the revenue issues we uncovered.”