By AAMER MADHANI and ZEKE MILLER

CHICAGO (AP) — President Joe Biden on Monday took his first flight aboard the modern VH-92A helicopter that serves as Marine One after years of delays to the program to replace the aging aircraft that carry the president and vice president.

Biden boarded the Sikorsky-made helicopter after arriving on Air Force One in Chicago where he is speak Monday evening at the Democratic National Convention. The maiden presidential voyage carried him from O’Hare International Airport to the parking lot of Chicago’s Soldier Field, often used as a landing zone for presidential travel.

It marked a crucial milestone in a two-decade-long process to replace the Vietnam-era helicopters that have been in use carrying presidents, in some cases, since the 1970s.

An initial effort to purchase new helicopters was kicked off by the Bush administration in an effort to improve communications and mission capabilities in the post-September 11, 2001 environment, but it was scrapped by President Barack Obama after rampant cost overruns.

The new program was started by the Obama administration, and the helicopter, branded the “Patriot” by its manufacturer, made its public debut during the Trump administration. But issues with the secure communications system on board — required so that the president can carry out classified conversations and make military decisions in an emergency — as well as a tendency to scorch the White House South Lawn led to years of delays to the program.

The Marine Corps reported last year that the communications issues had been resolved, but it was not clear whether modifications to the exhaust system have eliminated the threat to the White House grass.

Marine Helicopter Squadron One, which operates the aircraft, has been using the VH-92 helicopters for several years in regular service, including test flights around Washington, D.C. and to carry White House staff and security personnel. The final VH-92A helicopter was delivered earlier this month, giving the USMC 21 operational aircraft and two for testing. The total program cost for the 23 aircraft was about $5 billion.

The new helicopters are based on the commercially-available Sikorsky S-92 and are larger and have longer range than the older VH-3D and VH-60N models.

___ Associated Press writers Tara Copp and Lolita C. Baldor contributed to this report.