Theories

The aircraft made a sharp turn back over the South China Sea at 01:21 UTC — a manoeuvre consistent with manual input rather than a systems failure. It then flew along the border of Malaysian and Thai radar coverage, suggesting knowledge of radar range, before heading south into the Indian Ocean. The 2018 Malaysian Safety Investigation Report states the "change in flight path was the result of deliberate action by someone on the aircraft."

The leading suspect under this theory is Captain Zaharie Ahmad Shah, based on the flight's diversion coinciding with his shift in command, simulator data found on his home computer showing extended southern Indian Ocean routes, and the absence of any credible alternative actor. Malaysian police found no compelling motive, and the official investigation did not reach a conclusion on individual responsibility.

Supporting evidence

• Deliberate ACARS and transponder disabling sequence
• Navigated along waypoints suggesting pilot familiarity
• Simulator flight paths on Zaharie's home PC matching a remote Indian Ocean end point
• No structural failure, weather event, or known mechanical cause
• Confirmed debris scatter consistent with high-speed uncontrolled descent — not a controlled ditching

Under this scenario, a sudden depressurisation event (fire, electrical failure, or a structural breach) rapidly incapacitated everyone on board, including both pilots. The aircraft then continued on autopilot until fuel exhaustion somewhere over the southern Indian Ocean, after which it descended and impacted the water.

This theory is supported by similar precedent cases — Helios Airways Flight 522 (2005) flew on autopilot with an unconscious crew until fuel exhaustion. Proponents argue the lack of any distress call is more consistent with sudden incapacitation than with a deliberate act.

Supporting evidence

• No distress call or communication after transponder loss
• Helios 522 precedent for ghost-flight behaviour
• Aircraft on autopilot would maintain course without conscious input
• Does not require assigning intent or motive

Problems with this theory

• Does not explain the deliberate turn-back and radar-avoidance manoeuvre before communication loss
• ACARS was disabled before the transponder — hypoxia doesn't disable systems selectively

A catastrophic onboard fire or electrical failure could have destroyed communications systems and incapacitated the crew. Some analysts have pointed to the lithium-ion batteries in the cargo hold (a known fire risk) as a possible ignition source. Under this theory, the crew may have turned the aircraft around to return to Kuala Lumpur before the situation became unrecoverable.

Problems with this theory

• A fire severe enough to disable all communications would likely have caused structural damage inconsistent with the intact debris recovered
• The aircraft continued flying for approximately 7 hours — difficult to reconcile with a catastrophic fire
• The deliberate radar-avoidance manoeuvre is unexplained by an emergency scenario

Regardless of what caused the diversion, researchers have worked to narrow down where MH370 ended its flight. CSIRO oceanographer David Griffin's drift modelling — working backwards from confirmed debris found on Indian Ocean coasts — consistently places the impact point on or near the 7th arc between latitudes 25°S and 35°S.

Independent researchers Victor Iannello, Richard Godfrey, and the MH370 Independent Group have published analysis suggesting a specific end point around 33–35°S on the 7th arc, which falls within an area not yet searched. The debris scatter from the flaperon (Réunion), the outboard flap (Tanzania), and other confirmed pieces is consistent with a high-speed descent and ocean impact — not a controlled ditching.

Key data points

• Inmarsat satellite handshake arcs place the aircraft on a southern flight path
• CSIRO drift modelling is consistent with impact near 35°S, 93°E
• Debris fracture patterns on the flaperon indicate it was not set for landing — consistent with high speed at impact
• ~7,428 km² of the highest-priority zone has not yet been searched