London's air ambulance, known as the Helicopter Emergency Medical Service (HEMS), flies from a helipad above the A&E department at the Royal London Hospital in Whitechapel. A trauma doctor and an expert paramedic can reach an incident - and begin a patient's critical care - within minutes of the 999 call.
On the spot, the HEMS doctor can carry out procedures that normally take place in the hospital, including surgical procedures, strong pain relief administration, and anaesthetising patients with serious head injuries.
This ability to reach patients so quickly and deliver immediate intensive care has saved many lives. It's also meant that many people return to normal life after accidents that might otherwise have resulted in long term disability.
The HEMS helicopter flies approximately 1000 missions a year, operating seven days a week in daylight hours, and can reach anywhere within the M25 inside twelve minutes. Most incidents are road traffic accidents and falls from heights.