Flight 191 was historically the flight number for many notable fatal aviation accidents that occurred between the 1960s and 1980s, including:

Aeroflot Flight 191 (1963), crashed on final approach to Ashgabat International Airport, killing 12 people

X-15 Flight 191 (1967), or X-15 Flight 3-65-97, experimental test plane, broke apart in flight, killing its test pilot

Prinair Flight 191 (1972), crashed at Mercedita Airport in Ponce, Puerto Rico, killing five people

American Airlines Flight 191 (1979), crashed shortly after takeoff from Chicago O'Hare Airport, killing 273

Delta Air Lines Flight 191 (1985), crashed while on final approach to Dallas-Fort Worth, killing 137

These accidents, especially the American and Delta crashes, have caused 191 to be seen as an unlucky number by some in the aviation industry, and consequently several commercial airlines now specifically avoid using it as a flight number. Source :Wikipedia