Photo: Gust lock cable springs in aft equipment compartment, from Eddie's aircraft.
Your flight controls may be hydraulic but they have cables attached to them and they can be whipped about by the winds when the hydraulics are not pressurized. In that state, the wind can damage the flight control stops and internal components, so the gust lock is used to secure the rudder and ailerons in a neutral position and the elevator in a nose down position. The throttle is locked as well to prevent you from attempting to fly the airplane with the gust lock on.
While the gust lock is treated with hardly a few paragraphs in the Aircraft Operating Manual, it needs further research. Applying lessons from previous, non-Gulfstream, aircraft to flight control checks in your Gulfstream can be a big mistake. The worst I've heard so far: before taking the runway pulling the yoke full aft and allowing it to fall full forward in lieu of a 60 knot elevator free check. First: it is against flight manual procedure. Second: if the gust lock springs dedicated to the elevator and rudder should break, this could end up locking the elevator prior to takeoff. Yes, this scenario is extremely unlikely, but we often protect against the extremely unlikely. More about this below.
The better you understand the gust lock system, the better you will understand the need to set it when the hydraulics are depowered, the need to "unset" it before hydraulics are powered, the need to do a full flight control check prior to flight, and the need to do an "elevator free" check during the takeoff roll at 60 knots.
Unfortunately the pilot manuals we have are of little use when it comes to understanding the gust lock. I think we can piece together a better understanding with what we have at hand and maybe a little guess work too.
Caveat: I will be the first to admit that I fly paranoid and I might be crying wolf here. But the gust lock on this airplane is fault intolerant. That is, if it fails, it does not notify the pilot and could leave the airplane in an unflyable condition. Gulfstream designed the system with two springs to pull the gust lock off the rudder and elevator should the cable fail, two more springs do the same for the ailerons, and a fifth for the throttles. We don't know what will happen if these springs fail; it is possible, however unlikely, that vibration could move the applicable latches to the locked position. I think the following steps are critical for every flight:
There are a lot of poor techniques out there regarding what to do if you start the engines before disengaging the gust lock. Older Gulfstreams (GIV and earlier) had a method of removing hydraulic power from the flight controls. Some pilots would do this, disengage the gust lock, and then restore power to the flight controls. I've never seen this done but see how it can work. But it ignores the fact you could actually damage the lock, actuators, and the controls themselves. The Air Force G-III manual, the Technical Order 1C-20B-1, p. 2-53 has this to say: "If engines are started with the rudder locked, hydraulic pressure surge may break the lock and cause damage to the actuator or rudder." G450 AOM, §2A-27-80, ¶2B says "FAILURE TO ALLOW HYDRAULIC PRESSURE TO DISSIPATE PRIOR TO ENGAGING THE GUST LOCK MAY CAUSE DAMAGE TO AIRPLANE STRUCTURE." We all know that. So why wouldn't the same circumstances cause damage right after engine start? You need tohae the airplane inspected.
One final note, if you have any kind of speed on the tail the airloads will push up on the elevator and prevent the gust lock from disengaging. Who would be so stupid as to try this? See: Mishaps / Gulfstream IV N121JM. My advice: follow the checklist using a challenge-do-response method. If you find yourself with the engines running and the gust lock engaged, shut the engines down, cancel the flight, and have a Gulfstream mechanic inspect the airplane.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
[G450 Aircraft Operating Manual, §2A-27-80]
That last statement is wrong: it locks the elevator full nose down.
You need to actively move each control surface into the lock. In our GV this wasn't done with the rudder and it spent so much time being beaten by the wind it eventually wore down the latch of the rudder gust lock to the point it didn't work.
NOTE: The gust lock is effective in preventing flight control movement during wind speeds up to sixty knots (60 kts). If weather conditions are forecast to include stronger winds, consideration should be given to securing the aircraft within a hangar or other suitable shelter.
[G450 Maintenance Manual, §27-70-00, ¶1.A.]
There are indeed two springs for the rudder and elevator lock and two more for the ailerons. That should minimize the chance of inadvertent control locking if one spring fails, but not if the parts securing the springs fail. You could have a single aileron spring failed and not know it until the second spring goes. But you always have the rudder should this happen. It is absolutely critical that you inspect the rudder and elevator springs during every preflight.
NOTE: The gust lock system should not be engaged during taxi operations or ground runs, and for that reason there is an interlock between the gust lock handle and the throttle levers that restricts throttle lever movement with the gust lock system engaged.
[G450 Maintenance Manual, §27-70-00, ¶1.B.]
Figure: G450 gust lock controls, from G450 Illustrated Parts Catalog, §27-70-00, figure 1, sheet 1.
The gust lock cables run the length of the airplane from the handle in the cockpit to the rudder and elevator assembly in the tail. Along the way they lock the ailerons with an assembly forward of the wings that intercepts movement of the aileron cables. The throttle lock is handled by an axle and lever assembly from the gust lock lever to the throttle quadrant.
Photo: Gust lock cable and springs under elevator / rudder assembly, from Eddie's aircraft.
The gust lock cables are connected to two levers mounted to an axle which is common to the elevator and rudder gust locks. (Only the bottom cable is visible in this picture.) A pair of springs pull this axle from above to the off position, the bottom cable pulls it to the on position. If for any reason the cable is broken, the spring pulls the rudder and elevator locks to the off position.
Figure: G450 gust lock handle, from FlightSafety G450 Maintenance Training Manual, figure 27-104.
The handle in the cockpit locks into place with an internal latch and places tension on the "on side" cable. This places tension on the aileron lock, elevator lock, and rudder lock. The throttle lock is activated with levers on the gust lock axle. In each case, the individual lock is not engaged until each control is manipulated into the locked position and checked for proper locking.
CAUTION: Failing to ensure each control is locked can result in damage to the lock itself. For example, if the rudder is not locked it can be whipped about left and right through the lock itself, gradually wearing it away to the point it is no longer effective. This caution does not appear in any manuals, but we've seen this in actual practice on several airplanes.
CAUTION: You should check each axis for secure locking, including the throttles. There does not appear to be a regular inspection interval for the gust lock system, so it is up to you to make sure it works.
Figure: G450 gust aileron latch, from G450 Illustrated Parts Catalog, §27-77-00, figure 1, sheet 6.
[G450 Maintenance Manual, §27-70-00, ¶1.A.] The aileron gust lock mechanism is located at FS 283 below the cabin flooring.
The aileron lock is about the midpoint (location "K" in the drawing above and part number 355 here). The springs (part number 385 in the drawing) pull the lock off if the tension in the cable is relaxed for any reason.
Figure: G450 right aileron primary cable assembly, from G450 Maintenance Manual, §27-11-07, figure 409, sheet 1.
The gust lock aileron latch (shown above, part number 355) is at FS 283 and appears to lock the right aileron primary cable at a pulley located at FS 280, shown in this diagram.
Figure: G450 right aileron primary cable assembly, from G450 Maintenance Manual, §27-11-07, figure 409, sheet 2.
CAUTION: While the aileron lock is spring loaded to the off position, releasing the gust lock handle in the cockpit will only release the aileron cable if the springs are working. It is critical you perform a flight control check after engine start to verify the aileron lock has released. You have absolutely no other indication of spring failure. (The springs are not visible during preflight inspection.)
Figure: G450 gust elevator latch, from G450 Illustrated Parts Catalog, §27-77-00, figure 1, sheet 7.
The elevator lock is in back (location "L" in the drawing above and part number 455 here).
[G450 Maintenance Manual, §27-70-00, ¶1.A.]
Figure: G450 longitudinal control system, from G450 Maintenance Manual, §27-30-00, figure 503, sheet 3.
The elevator gust lock (part number 455 above) can be seen in this diagram in the unlocked position. To lock it, tension on the gust lock cable overcomes tension on the springs to push the lock upward into a pin that is only within its grasp when the elevator is 13° down. If both gust lock springs are broken we do not know which direction the lock will gravitate: up or down. It could be that other components mechanically linked may push the handle up. (We just don't know, so I am building a worst case scenario because . . .)
[G450 Aircraft Operating Manual, §2A-27-20, ¶1.] The deflection range of the elevator is twenty- four degrees (24°) up and thirteen degrees (13°) down.
CAUTION: The locked position of the elevator, 13° down, is the same as its natural position with no air loads. That is, without any airspeed the weight of the elevator itself drives the elevator to the down position. If the gust lock springs are broken, vibration could conceivably move the elevator gust lock to the on position, even after a valid flight control check. A poorly conceived technique is to pull the yoke full aft and then allow it to fall forward just prior to taking the runway for takeoff. If the spring is broken this could conceivably lock the elevator just prior to takeoff. For these reasons, it is absolutely critical the elevator be checked for freedom of moment once normal air loads have been established. The AFM requires this check be done at 60 knots.
Note: our manuals do not provide any information on what to suspect in the event both these springs fail. I have given a "worse case scenario" here just to emphasize that the lock could engage itself without these springs. You really ought to check for the presence of both springs during your external preflight and you really ought to check the elevator free at 60 knots.
Figure: G450 gust rudder hook, from G450 Illustrated Parts Catalog, §27-77-00, figure 1, sheet 7.
The rudder hook is in back (location "L" in the drawing above and part number 580 here).
[G450 Maintenance Manual, §27-70-00, ¶1.A.]
CAUTION: You should inspect both gust lock springs in the aft equipment compartment to ensure you do not have an inadvertent rudder lock engagement caused by vibration of this lever. There is no aircraft manual warning about this and it may not be a problem. But in the absence of more information from Gulfstream, we do not know how the rudder lock will behave in the event both springs fail.
Figure: G450 gust rudder control rod and link, from G450 Maintenance Manual, §27-21-09, figure 3, sheet 1.
I believe the rudder hook (part number 580 above) is meant to capture the paddle on the radius shown here.
Figure: Electrical throttle connections, from FlightSafety Maintenance Training Manual, figure 76-4.
[G450 Maintenance Manual, §27-70-00, ¶1.A.] A mechanical interlock within the cockpit center pedestal prevents advancing the power levers more than six percent (6%) forward of ground idle if the GUST LOCK handle is engaged. The blocking action of the interlock cannot be overcome with manual force to advance the power levers.
Figure: Throttle gust lock spring, from "Some guy" in tech ops.
The G450 throttle quadrant has one and only one mechanical linkage with the rest of the airplane and that is for the gust lock. While the system isn't described in any of the pilot or maintenance manuals, it appears the gust lock handle is directly linked to an axle just forward of the throttle quadrant with a torsion spring that pulls the throttle gust lock, and therefore the gust lock handle itself, to the off position if tension on the gust lock cable is lost for any reason.
Figure" Throttle gust lock internals from "Some guy" in tech ops.
An arm on the axle forward of the throttle quadrant lifts a spring lever arm which locks the throttle from inside the throttle quadrant.
FSI G450 MTM, FlightSafety International Gulfstream G450 Maintenance Training Manual, August 2008
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013
Gulfstream G450 Illustrated Parts Catalog, Revision 17, October 31, 2012
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Technical Order 1C-20B-1, C-20B Flight Manual, USAF Series, 1 November 2002