From a safety standpoint, the value of any companion lies in its accuracy and currency. An unofficial PDF that has not been validated against the latest FCOM revisions poses a risk. Therefore, professional pilots treat “cockpit companions” as study aids for ground training or sim sessions, not as primary references in flight.
The by Bill Bulfer is considered the definitive "pilot-friendly" guide for mastering the complex systems of the Boeing 737 aircraft series. Whether you are a professional pilot preparing for recurrent training or a dedicated flight simmer, this manual provides depth beyond standard airline training materials. b737 cockpit companion pdf