AIN Blog: Trouble With Gust Locks

 - June 19, 2014, 11:13 AM

It is way too soon to speculate about what might have caused the Gulfstream IV runway excursion crash at Hanscom Field in Bedford, Mass. on May 31, but the NTSB preliminary report’s focus on the gust lock system raises some questions.

First, are gust locks even necessary on a large business jet like a GIV? The answer is yes, because there is nothing to hold the flight control surfaces still if strong winds are blowing, especially if the airplane is parked with its empennage facing the oncoming wind. Larger airplanes with hydraulically actuated flight controls aren’t subject to gusty wind damage because the hydraulics lock the control surface into position once the engines are shut down. This is also the case for fly-by-wire flight controls.

Interestingly, in a search of FAA accident/incident and NASA Aviation Safety Reporting System (ASRS) databases, it seems that even with gust locks engaged damage can happen to flight controls. In December 2007, the pilots flying a Chautauqua Airlines Embraer ERJ-145 aborted the takeoff after they noticed that there was no response when they applied aft pressure to the yoke. They were able to stop in time and returned to the gate. A subsequent examination of the airplane found: “Damage to the fairing cutout area was observed. After removing the fairing, both elevator control tubes were found broken along with broken damaged stop fittings, elevator skin damage on the lower surface.” The report also noted, “This aircraft sustained high-gust winds for a period of 20 hours prior to takeoff while parked with gust locks reported to be engaged while RON at JFK.”

A Citation 550 pilot found out what can happen if the control lock is not fully disengaged in this 1983 incident: “Fire erupted in control pedestal on takeoff. Control lock not fully off. Shorted the thrust reverser micro switch.”

Non-standard control locks have always been a problem, illustrated by this incident in a Piper Cherokee Six in 1982: “Pilot forgot to remove Allen wrench used as a control lock. Aborted. Ran off end of ice- and snow-covered runway.”

What’s odd is that some aviators apparently feel compelled to engage the gust locks while taxiing, which as far as I can tell isn’t a procedure mentioned in any flight manuals. Jet manuals that I checked instruct pilots to remove the control locks during preflight and install them after shutdown, but nothing about while the airplane is moving.

In this ASRS report, a Citation Sovereign pilot tried to engage the gust lock while taxiing in a strong wind: “It was very windy (gusting at times to over 40 knots) with shifting winds and heavy rain. I was the pilot taxiing the airplane. Original clearance was to taxi to 19 via Bravo and cross 14. I understood the instructions and looked at my iPad. We began taxiing on the [FBO] ramp. Bravo parallels 19 but also does a 90-degree turn and connects to the departure end of 19, which is where the ramp comes out. As we crossed the ramp I was struggling to re engage the gust lock due to the wind and had my partner help push on the rudder to help engage it.” As a result of these actions the pilots accidentally strayed onto a runway that they weren’t cleared to enter.

The compulsion to lock the controls during taxi for takeoff apparently is not recent, as this example from an online forum indicates. “I had a friend killed in a C-47 because of this. They taxied out in some pretty bad weather and had the crew chief get out and put the elevator lock in place while they taxied a mile or more downwind to get to the end of the departure runway. They then took off with the elevator still externally locked, and there is no way to remove it from inside. The [instructor pilot] had more than 20,000 hours, his student had about 10,000 and the airplane didn’t care–it just did what they told it to. On takeoff it pitched up steeply. They probably realized what they’d done at this point and sawed the power off. The nose dropped, right straight ahead, and they put in a blast of power to catch the sink. They made a valiant effort but it still pancaked in hard enough to kill all three of them. The crash became a safety-school example of the dangers of ‘interrupted habit patterns’ in that we always do a control check before departing the ramp–right? Then they short-circuited that check by re-installing an external lock after that check. Add a hurry to depart in advance of worsening weather and you have a recipe for this.”

Finally, one bored and incredibly lucky charter pilot returning on a ferry flight in Alaska answered the question about what happens when the control lock is engaged during flight, in this ASRS report: “Last flight of a long but unprofitable day. Flying back home in an empty airplane in excellent weather. I got a little bored and wanted to see what would happen if I stuck the gust lock into the controls in flight. The airplane went into a 300 fpm descent and this descent rate was increasing. The ground was coming up to meet me and the controls were completely locked. The increase in airspeed caused a higher airflow over the horizontal stabilizer. This forced the yoke back and locked the gust lock (control lock) into place and it could not be pulled out. All this time the airplane is in a straight descent for the ground with the engine running at full cruise power. I could not pull up, turn or even increase the descent. But by pushing forward on the yoke and holding the control lock I was able to line up the two control lock holes to where I could pull out the control lock. In all, I lost 300 feet. I was lucky. When I did this, I did not think of what might have happened. I do not recommend trying this maneuver.”

None of this is meant to second-guess the NTSB investigators who are working to figure out what happened with the GIV in Bedford, but it does appear that the investigators purposely released information in the preliminary report to help pilots understand the important of checking for free movement of flight controls before takeoff. Gulfstream issued a maintenance and operations letter on June 13 to all Gulfstream operators to “remind flight crews of the importance of adhering to flight procedures published in applicable Airplane Flight Manuals (AFM) to confirm flight control integrity and freedom of motion.” It added that crews should perform the following as set forth in the applicable AFM procedures: “ensure the gust lock is off before starting engines (not applicable for G650); check flight controls for freedom and correct movement before taxi/takeoff; and confirm the elevators are free during the takeoff roll.”