Auto-Throttles for Challenger 604

Safe Flight Instrument and Bombardier this past Friday earned an STC for a limited authority auto-throttle system on the Challenger 604. The lack of ATS on the 604 has been a glaring shortcoming, considering that its heavy-iron class arch rivals, the Gulfstream IV-SP and Dassault Falcon Jet 900EX, both have auto-throttle systems. On the 604, however, the ATS is a standalone system that is not integrated with the FMS or a full performance management computer.

The Safe Flight ATS is designed to be used only with the autopilot engaged. As such, it is restricted to inflight use in accordance with autopilot operational limitations. For example, it cannot set takeoff thrust and it cannot be used on precision approaches below 80 feet agl.

After takeoff, the flight crew first engages the autopilot and then the ATS. The ATS has two operational modes: maximum continuous thrust hold and indicated speed/Mach hold. MCT hold is the primary, or default, ATS mode when first engaged. The ATS computer adjusts MCT for ambient conditions and bleed air drains according to digitized tables from the AFM. Right-side master and left-side slave servos drive the throttle levers to maintain MCT throughout the climb.

After leveling off, the ATS will maintain MCT until the crew selects the indicated speed or indicated Mach hold mode. Speed/Mach selection is done by using the speed set knob on the Challenger 604's DFCS glareshield control panel. The system will maintain speed within plus/minus four knots of the pre-selected speed, which, coincidentally, is the width of the airspeed bug on the Rockwell Collins Pro Line 4 PFD airspeed scale.

Pilots will appreciate the smooth, slow throttle adjustments made by the ATS. It's not frenetic and alarming to passengers as are some other auto-throttle systems installed in business aircraft. In addition, the crew can override the servos in the event that manual thrust adjustments are needed.

On descent, the crew must select DFCS VNAV or vertical speed hold mode. ATS is not available in the flight level change (FLCH) mode. The ATS then holds the selected speed in the descent.

If the crew selects too low an indicated speed at any time, the Safe Flight ATS has a low-speed, or high angle-of-attack, protection system that prevents stall. The AoA protection system has flap configuration logic.

The Safe Flight ATS consists of an auto-throttle computer, dual throttle servos, annunciators and switches. It's priced at $95,000 and will be a factory standard option starting at serial number 5400. Additionally, it can be retrofitted to any Challenger 604. Previous Challengers can't be retrofitted because of avionics incompatibilities.

By Fred George