HISTORY OF FLIGHT On May 28, 2010, about 1220 eastern daylight time, a Cessna 560XL, N662QS, incurred minor damage when the main cabin door opened while in flight, shortly after takeoff from Westchester County Airport (HPN), White Plains, New York. The certificated airline transport pilot-rated captain, first officer, and the two passengers were not injured. Visual meteorological conditions prevailed, and the flight was operating on an instrument flight rules flight plan. The corporate flight was destined for Nantucket Memorial Airport (ACK), Nantucket, Massachusetts. The flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91K. The captain and first officer were interviewed separately by Federal Aviation Administration inspectors following the incident. Both pilots were qualified to act as captains in the Cessna 560XL, and their roles for the incident flight were previously defined by the operator. The captain was also designated as the pilot flying, while the first officer was designated as the pilot not flying (or pilot monitoring). During his safety briefing with the passengers after boarding, the first officer closed the cabin door. The first officer remembered rotating the door locking handle, but could not recall the direction he rotated it. He also could not recall looking at the door post locking indicator lights or the cockpit indicator. After completing the required checklists, the crew taxied the airplane "slowly" toward runway 16, and was delayed before being cleared to depart. During the delay, the captain focused his attention on the air traffic control communications and formulated a mental picture of the traffic in the area of the airport. Just after takeoff, the captain recalled that he heard the cabin door leaking and the first officer observed that the DOOR SEAL/CABIN DOOR annunciator light was illuminated. When asked, the first officer did not recall the master caution annunciator illuminating. The crew requested and was cleared to return to runway 16. While on the downwind leg of the traffic pattern, the main cabin door opened. The crew subsequently declared an emergency, and landed uneventfully with the main cabin door open. AIRCRAFT INFORMATION According to the Cessna 560XL maintenance manual, the cabin door was located on the left forward side of the airplane. The door was hinged at its bottom, opened outward, and was equipped with a pre-catch mechanism, which assisted in holding the door closed before the locking mechanism was engaged. Eight locking pins were mechanically linked to internal and external lock/unlock handles, with three locking pins located in each side of the door and two locking pins at the top of the door. When actuated by the lock/unlock handle, the door pins extended into the door frame sockets to secure the door in the closed position. Six electrical switches monitored the door's closed and locked position, with four switches actuated by the upper and lower side locking pins, one switch monitoring the door vent, and one switch monitoring the door locking mechanism's position. The switches were electrically connected to the warning annunciator panel in the cockpit, and only when each of the six switches was closed, would the CABIN DOOR annunciator extinguish. Should any of the six switches toggle from locked to unlocked, the CABIN DOOR and MASTER CAUTION annunciator would illuminate. Resetting the MASTER CAUTION would extinguish the MASTER CAUTION annunciator, but the CABIN DOOR annunciator would remain illuminated. A DOOR LOCKS panel was also located on the cabin door post, and was equipped with six light-emitting diodes to indicate the position of the door locks, door handle, and vent door. Each light illuminated when its respective switch sensed that a component was in the unlocked position and extinguished when its respective component was in the locked or closed position. When the door was closed and locked, the door vent light extinguished after several seconds and the vibration from the closing vent could be felt near the bottom of the cabin door. Additionally, nine indicator windows were placed throughout the door, so that when the door was properly closed and locked, a phosphorescent green flag was clearly visible through each respective indicating window. The cockpit annunciator panel provided numerous system notifications to the crew. The CABIN DOOR annunciator light (colored yellow) was located on the bottom of three rows of annunciator lights, on the sixth column from the right. The GROUND IDLE annunciator light (colored white) was also located on the sixth column from the right, above the CABIN DOOR annunciator light, on the first row of annunciator lights. The GROUND IDLE annunciator would normally be illuminated after engine start and while the airplane was taxiing to the runway, and would be the only annunciator illuminated just prior to takeoff. PERSONNEL INFORMATION The captain held an airline transport pilot certificate with ratings for airplane single and multi-engine land. The pilot's most recent first-class medical certificate was issued in July 2009. He reported 7,459 total hours of flight experience, 1,737 hours of which were in the incident airplane make and model. The first officer held an airline transport pilot certificate with ratings for airplane single and multi-engine land. His most recent first-class medical certificate was issued in April 2010. He reported 5,964 total hours of flight experience, 1,432 hours of which were in the incident airplane make and model. METEOROLOGICAL INFORMATION At 1156 recorded weather at HPN included calm winds, a broken ceiling at 3,200 feet, 10 statute miles visibility, temperature 19 degrees C, dew point 11 degrees C, and an altimeter setting of 29.98 inches of mercury. FLIGHT RECORDERS The airplane was equipped with a cockpit voice recorder, which was retained and auditioned in the NTSB Vehicle Recorders Laboratory. Audition of the recorder revealed a series of events consistent with the flight crews' statements. While completing the engine start checklist, the crew noted 'cabin door closed' and 'lights out'. Shortly thereafter, the flight crew noted that the annunciators and instruments were checked. The flight crew completed the runway items checklist and noted "lights, heat, and lights out." TESTS AND RESEARCH Two FAA inspectors examined the main cabin door after the incident. According to the inspectors, the door had been over-extended when it opened in-flight, and had scraped against the runway during the landing. Damage to the door precluded closing the door normally, though the door could be opened and closed with force provided by individuals on the inside and outside. Detailed mechanical inspection of the door revealed that the pre-catch mechanism appeared to operate normally, and there was no visual indication of any door locking pin or pin socket failure. The annunciator lights on the DOOR LOCK panel and the CABIN DOOR annunciator functioned normally, including illumination of the MASTER CAUTION annunciator when the door was opened. The inspectors further noted that if electrical power (from a the airplane's battery or a power cart) was applied to the airplane with the cabin door open, the CABIN DOOR annunciator remained steadily illuminated until the door was closed. The illumination of a MASTER CAUTION annunciator, accompanied by a blinking CABIN DOOR annunciator, would only occur after the door was closed, locked, then unlocked and reopened. ADDITIONAL INFORMATION According to the operator's Expanded Normal Procedures, Engine Start checklist, both of the pilots were to perform the following actions: Cabin Door.................Closed/Light Out -Close and latch cabin door -Verify cabin door pin indicators are visible -Verify cabin door red LED lights are extinguished -Verify the cockpit Cabin Door annunciator is extinguished. The Runway Items flow checklist procedure required the pilot monitoring to ensure that all annunciator lights were extinguished with the exception of GROUND IDLE after being cleared for takeoff.