Autothrottle Differences

If you have time in previous Gulfstreams with autothrottles, you will be used to pushing the throttles forward somewhat (GV/G450/G550: "the EPR values of both engines are greater than 1.05 EPR") and pressing one of the engage switches aft of the throttle stems, making sure you do so before 60 knots. The key was achieving the EPR prior to hitting the switches. That doesn't work in the GVII. What does the book say about this?

The PAS is fairly cryptic: "A/T ENG / DISENG button — Push either with thumb — Auothrottles (AT) → ON if engagement criteria met" but then it doesn't say what those criteria are. The "Ground and Flight Operations" section of the Operating Manual (02-01-10, p. 12) is what we are looking for:

• The green TO annunciation indicates that the Takeoff Thrust Control Mode is engaged where the Auto-Throttles are being commanded to the Takeoff or FLEX N1. The Auto-Throttles engage in the TO mode when the thrust levers are manually advanced above 19-degrees TLA, the airspeed is below 60 knots, N1 split between engines is less than 10%, and one of the A/T engagement switches are depressed.

• The white TO armed annunciation (left side of the center FMA) indicates that the Takeoff Thrust Control Mode has been armed. It’s automatically armed on the ground when:

  1. At least one engine is running

  2. Guidance panel speed target is set

  3. Thrust reversers are stowed

  4. Valid ADS and IRS sensors selected on PFD source

  5. FCCs in normal mode

  6. FADECs are valid

  7. Engine not in alternate mode

This always worked in the simulator but in our airplane it only worked as advertised for the first month but then we had times when the autothrottles would not engage during takeoff. We took videos of everything and Gulfstream cheerfully replaced our throttle quadrant. A few weeks later the problem returned. More videos.

Turns out it was operator (us) error but our engines bear some of the blame. The criteria says you need at least 19-degrees Throttle Lever Angle. (The PAS and many Gulfstream manuals favor the term TLA, Throttle Lever Angle, but do use the term TRA, Throttle Resolver Angle also.) That doesn't seem like much, but if you get into the CMC you can get the angle and it turns out 19-degrees is almost halfway forward.

So we became very careful to push the throttles up at least that far, but we did so smoothly. That only made matters worse and we rarely got them to engage correctly. More videos. Then Gulfstream said we had more than a 10% N1 split between engines when we hit the engage button. But on the video that happened with the left engine at 42% and the right at 47%, only five percent difference! Wrong, it was a 100(47-42)/40 = 13%. More video.

So I put a camera on the jump seat and made a conscious effort to push the throttles quickly (nothing smooth about it) to a little more than halfway. As it turns out, we have a considerable lag in our left engine going through 50% or so. According to Pratt, the engines have 5 seconds to make it to the target N1 and as long as they both do that, there is no given tolerance for having them spool up in sync. So what to do?

Unlike the older Gulfstreams, the autothrottles don't care about a minimum N1 at engagement; only that the engines are within 10 percent of each other. We figured that as long as we have the throttle lever angle satisfied, we can engage the autothrottles and let the engines spool up after we hit the switch. Here is a video of that technique: GVII Autothrottle Technique (New)

The pilot pushed the throttles almost to the target setting and pushed an autothrottle engage switch when the left engine was still at 24.5% and the right was at 25.2%, a 0.7% reading difference and 3% between engines. Into the 30% range the right engine raced past the left and the largest difference was when the left engine was at 48.3% and the right was at 58.3%, a 10.0% reading difference and 21% between engines.

Lesson learned: if you wait for some spool up and your engines are not well matched, you will be outside the split tolerance of 10% between engines and the autothrottles will not engage.

Now if your engines are perfectly matched this will not have been a problem for you. But if you have been hit and miss with the autothrottles, give this technique a try. For us, it is yet another item of muscle memory that needs relearning in this airplane.