The Towering Inferno

Architect Doug Roberts returns to San Francisco for the dedication of the Glass Tower, which he designed for owner James Duncan. At 138 stories (1,800Â ft/550Â m), it is the world's tallest building. Shortly after his arrival, an electrical short starts an undetected fire on the 81st floor. While Roberts accuses the building's electrical engineer, Roger Simmons of cutting corners, Simmons insists the building is up to standards.
During the dedication ceremony, public relations chief Dan Bigelow is ordered to turn on the tower's exterior lights to impress the guests and dignitaries. The lighting overloads the electrical system and Roberts orders it shut off. Smoke is seen on the 81st floor and the San Francisco Fire Department is summoned. Roberts and engineer Will Giddings go to the 81st floor but fail to prevent a security guard opening a door, leading to a fire flash which burns Giddings (and who is later disclosed to have died).
Roberts reports the fire to a dismissive Duncan, who refuses to order an evacuation. Ill-equipped firefighters arrive to tackle the blaze, which soon spreads out of control. SFFD Chief Michael O'Halloran forces Duncan to evacuate the guests in the Promenade Room on the 135th floor, directing them to express elevators. A guest, Lisolette Mueller, who is being wooed by con man Harlee Claiborne, rushes to the 87th floor to check on a young family. Simmons admits to Duncan that he cut corners to stay under budget.
Duncan, aided by Senator Parker and Mayor Ramsey, directs the elevator evacuation from the Promenade Room until the fire renders the express elevators unsafe. Not heeding Duncan's directions, one last group takes an elevator to the ground, but are killed when it stops on the fire's floor. The fire also traps Bigelow and his secretary/mistress Lorrie in his 65th floor office, killing them.
Security Chief Harry Jernigan and Roberts rescue Mueller and the family from 87. Jernigan gets the mother out safely but Roberts, Mueller and the two children are halted by a collapsed stairwell. They make a perilous climb down the stairs but fire forces them up to the Promenade Room. With fire suppression efforts becoming ineffective, the building loses electrical power and O'Halloran's men are forced to rappel down an elevator shaft.
A rooftop rescue results in disaster as guests rush the helicopter, causing it to crash, setting the roof ablaze and rendering further rooftop rescues impossible. Naval rescue teams attach a breeches buoy to the adjacent Peerless Building and rescue a number of guests, including Duncan's daughter, Simmons' wife Patty. Roberts rigs a gravity brake on the external scenic elevator allowing twelve people, including Roberts' girlfriend Susan, Mueller, and the children, one trip down. An explosion leaves the descending elevator hanging by a single cable at the 110th floor, where Mueller falls to her death. O'Halloran and others save the rest.
Simmons tells Duncan that he and others will use the breeches buoy next, but Duncan punches him, saying the rest will draw numbers but he and his son-in-law will be the last to leave. The fire reaches the Promenade Room and Simmons forces his way onto the buoy, leading to a struggle. Parker is pushed to his death by Simmons, who is killed by an explosion moments later.
An SFFD deputy chief summons O'Halloran with a plan to explode the million-gallon water tanks atop the building to extinguish the fire. Knowing it could result in his death, O'Halloran meets with Roberts and they set C-4 on the six water tanks on the 138th floor. They return to the Promenade Room, where the remaining guests tie themselves down. O'Halloran, Roberts, Duncan, Claiborne and most of the party-goers survive as water rushes through the building, extinguishing the flames.
On the ground, Claiborne learns that Mueller did not survive and is heartbroken. Jernigan gives him Mueller's pet cat. Duncan consoles Patty over her husband's death but does not disclose the cowardly way Simmons died. Looking up at the tower, Duncan promises that such a tragic debacle will never happen again in the future. Roberts says to Susan that he does not know what will become of the building, and perhaps it should be left in its fire-damaged state as "a kind of shrine to all the bullshit in the world". Informing Roberts that the casualty toll numbered less than 200, O'Halloran says they were lucky. Roberts agrees to consult with fire officials in the future when such buildings are designed. O'Halloran drives away, exhausted.

Question: Why does the helicopter crash?
Answer: guests rushed it