A terrible medical helicopter crash in Maryland on September 28 killed four of the five people onboard.

The chopper wasn’t equipped with a terrain system that might have warned the pilot that he was flying too close to trees, a member of the National Transportation Safety Board (NTSB) told the Washington Post. Investigators are also examining whether a mechanical problem was partly to blame for the crash.

The helicopter was en route from a car accident to a hospital in rainy, foggy weather when the aircraft went down. The pilot, the onboard paramedic, a volunteer medical worker, and a patient were killed. A second patient survived the crash, according to the Post.

Prior to this accident, this year had seen at least 19 people die in eight crashes, which involved nine medical helicopters, according to past reports.

All regular and platinum subscribers to the Hospital Safety Center can read tips for preventing medical chopper mishaps on hospital landing pads by logging in and searching for "helicopter."