A medical transport jet carrying a child patient, her mother and four others slammed into a Philadelphia neighborhood and exploded in flames.

The child had been treated in Philadelphia for a life-threatening condition and was being transported home to Mexico, according to Jet Rescue Air Ambulance, the plane’s operator. The flight’s final destination was Tijuana after a stop in Missouri. All six people aboard were from Mexico.

The plane crashed Friday 30 seconds after takeoff from Northeast Philadelphia Airport, which primarily serves business jets and charter flights.

Here’s what to know about air medical transport:

What is an air ambulance?

An air ambulance provides transportation to people in critical or life-threatening situations when a ground ambulance can’t reach a patient or wouldn’t get to them fast enough.

They help boost a patient’s odds of surviving and recovering, particularly in rural areas that don’t have trauma or burn centers, according to a 2017 U.S. Government Accountability Office report.

They’ve grown in importance as more rural hospitals have closed, according to a medical journal report from 2022.

Air ambulances can be either a helicopter or a fixed wing aircraft. Helicopters are generally used to take patients between hospitals or from the scene of an accident to a hospital. The airplane versions fly longer distances between airports.

Helicopters make up 74% of all air ambulances, according to the GAO report.

The Philadelphia crash involved a Learjet 55, a small business jet.

Who rides air ambulances?

People with traumatic injuries, pregnancy complications, heart attacks, strokes and respiratory diseases are the most common users of air ambulances, according to the National Association of Insurance Commissioners. More than 550,000 patients in the U.S. use them every year.

In Hawaii, for example, they are frequently used to take patients from less populated islands, where health care is more limited, to Honolulu, where the state’s biggest hospitals are located.

The owner of the plane that crashed in Philadelphia, Jet Rescue, flew baseball Hall of Famer David Ortiz to Boston after he was shot in the Dominican Republican in 2019.

What are they equipped with?

Air ambulances will often have similar life-saving equipment like ventilators and blood transfusion devices. They will have stretchers and incubators.

The Mayo Clinic, for example, said its air ambulances will have an external defibrillator, an external pacemaker and more than 60 medications.

Paramedics and emergency medical technicians care for patients on board. Sometimes doctors and nurses are on board.

What does an air ambulance cost?

The average air ambulance trip is 52 miles (84 kilometers) and costs between $12,000 and $25,000 per flight, according to the National Association of Insurance Commissioners. The medical condition of the patient and the staff on board can influence the price.

Many insurers will pay what they believe to be a reasonable but sometimes they will disagree with the air ambulance provider and in these cases the patient may have to pay the difference.

Insurance experts say big invoices are becoming more common as costs rise and coverage shifts.

Have there been other fatal crashes with air ambulances?

Yes, there were 87 accidents that led to 230 deaths from 2000 to 2020, according to a study in the journal Prehospital and Disaster Medicine. Nearly three-fourths of those accidents were on helicopters.

Human factors that can include pilot error or disorientation, impairment and fatigue contributed to 87% of the fatal crashes.

The Philadelphia crash was the second fatal incident in 15 months for Jet Rescue. In 2023, five crewmembers were killed when their plane overran a runway in the central Mexican state of Morelos and crashed into a hillside.