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Gulfstream VNAV Logic during Takeoff with NG-FMS

Gulfstream VNAV Logic during Takeoff with NG-FMS

Editorial - In 2018, I wrote a series titled “Everything you wanted to know about VNAV”. It was a multi-part series covering use of VNAV by phase-of-flight, from takeoff to landing. The link above will take you to the Pilot Gateway, where the entire pdf can be downloaded. In section 2, I addressed the practice of engaging VNAV prior to departure. All of which was true at the time but as things evolve, changes made the system more helpful to the crew. This article should be thought as an update to the original.

The original EPIC and NZ VNAV logic was such that if it was selected on the ground, it would immediately engage VIAS but would disable the takeoff logic crucial for departure. It was great for scenarios with an early climb constraint but because the system would bypass the takeoff logic, it would target the first FMS speed it encountered, which for most operators was 200 knots. Because the airplane wasn’t climbing or descending, it would then level the command bars, cancelling the takeoff pitch that was generated by pressing TOGA to a 0º pitch until the aircraft had rotated and could target 200 knots.

With Next Generation (NG) FMS, there were numerous improvements made to the VNAV logic. It has continued to be improved over time with several changes that have been put into practice by some training providers and flight departments, while others are hesitant because they were not addressed in the Pilot’s Guides or official guidance. The bullet points below show the high-level summary of the behavior:

  • G500 / G600 & G650 Block 3 –

o  Selecting VNAV prior to takeoff will illuminate the button on the guidance panel,

o  VNAV will appear in standby (armed) in white on the PFD,

o  Flight Director will maintain takeoff pitch attitude commanded by TOGA,

o  After takeoff, reaching 400 feet (with landing gear retracted), VNAV will become active changing from white to green VIAS on the PFD.

G500 / G600 with VNAV selected prior to takeoff

  • G650 Blocks 1 & 2 -
    • Selecting VNAV on ground works similarly to EPIC and NZ,
    • VNAV button will illuminate of guidance panel,
    • VIAS will activate on PFD,
    • Flight Director will reset to level (or almost level),
    • During takeoff roll, FD will begin to pitch up as acceleration is sensed. This is not the takeoff pitch commanded by selecting TOGA but a result of acceleration being detected by the AFCS.

G650 Block 2 with VNAV selected prior to takeoff

In all cases, the VNAV button can be selected on the guidance panel while on the ground. The primary difference is in G650 Block 1 and 2 - VNAV will become active immediately. G500 and G600 (and G650 when Block 3 is released) will behave differently - selecting VNAV on the ground will arm it but will maintain the takeoff pitch and logic until reaching 400 feet (along with gear retracted), at which point VNAV will switch from armed to active.

VNAV functionality during takeoff phase has been widely misunderstood. This has led to some mixed operating practices and word of mouth explanations of how the system works. This article is not meant to authorize or recommend the procedure. It’s merely a frame of reference for the crew to understand how the system behaves with NG FMS. As always, follow the guidance of the OEM and your flight department.

For questions contact the Flight Technical Services Team at