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The Fokker 100 "Gustlock" / Flight Control Lock handle: Why I consider it a possibly lethal safety hazard during landing gear problems.

A training event once happening triggered this consideration.

Gemakshalve is deze pagina alleen in het Engels geschreven.

This page is written using English language only.


This "casus" is about a Fokker 100 cockpit feature, which, under certain circumstances, could lead to an aircraft crash.

It should be noted: This can only happen if a pilot would make a big mistake.

Digging my "old stuff" I came across two old letters I still have from my time working at Fokker Aircraft.

I decided to do something with it, creating this text, hoping it is interesting for the audience.

I am NOT addressing the chance of a crash, only pointing the possibility DOES exist because of a situation I ever encountered in a training device.

One has to keep in mind, accidentally "using a wrong button", in various ways, did and does happen, many people just intermix left and right just calling out map directions to somebody at the steering wheel of a car...

The papers I found (see elsewhere in this article) helped regaining my memory to be able to describe what it was about.

I have no data anymore to back my claims, so I can not guarantee my technical information is 100% correct.

There is no other text regarding this issue on the internet as of date january 2024, therefore I will regard anything else found on the internet about it as copyright infringement, if used without consent.


A possible very dangerous situation can happen in a Fokker 100 if it is decided to use the alternate landing gear handle if normal gear extension does not work.

Potentially, a pilot could use the wrong handle and operate the "Flight Control Lock" handle, instead. Some time is lost before the decision is made.

This handle on the left side of the pedestal locks the "CONTROLS", like ailerons and elevator tip and even influences the throttles, so there is no way to steer the aircraft.

Because afterwards the landing gear AGAIN seems not to be extending properly, more time is lost, trying to interprete why also the Alternate landing gear does not work.

Now, the locking of the controls may not be immediately detected, it only will be detected NOT EARLIER than when the autopilot tries to trim or steer the aircraft.

AGAIN, precious time will be lost, the aircraft descending further and further, the autopilot will disengage providing the cavalry charge warning sound, but the pilot flying will be unable to regain control even hitting the FMP A/P disconnect bar.

The crew will be aware having additional problems, but having TWO major events which may be confusing at the same time during the approach, raises the following question:


If YES, they will survive...

Observation showing the problem is real.

Being employed as a "flight simulator engineer", supposed to keep the electronic equipment up and running, I was on duty when the Fokker 100 "cockpit system trainer" ("F100 CST") device was used.

This device was an operational cockpit lookalike, it did not have a motion, or visual system, and its takeoff and landing behaviour was simulated as a fixed "pathway" by software below something like 200 ft AGL altitude.

Above this simulated altitude a real "flight mode panel" (FMP) did control software copies of the flight augmentation and flight control computers for "flying" using the various autopilot modes and displaying on real F100 aircraft EFIS/MFDS displays.

The device could "navigate" also, it contained a real Flight Management Computer as well, for the autopilot Navigation and Profile modes.

One day, at the end of a training session I received a flight instructor's complaint like: "The alternate landing gear handle does not work, the autopilot failed in the end and the thing crashed, probably you have to reload the computer."

I checked, and found the flight control lock on, locked, and the alternate gear handle in its normal position. The "CST" behaved normally and there was no reason to reinitialize the computer software.

Assuming the complaint to be invalid, I talked to the same instructor re-appearing, the next day.

He told me he did NOT put the handles back but left the CST as it was, enabling me to troubleshoot.

This backed my conclusion they operated the wrong handle, as I expected both handles the other way around. I told about the situation and I still remember he agreed this was some "oops" moment, and this indeed could be dangerous if happening in the real, as I thought.

IF such happens during training, the pilots are NOT aware, the instructor is NOT aware, why think it could NOT happen in the real?

It kept me puzzled and I decided to do something with the knowledge.


Trying to get the problem noticed, somebody suggested "write it down and put it in the companies ideas box", and so I did.

One of my suggestions was an automatic mechanical lock of the flight control handle. In the end, there is one using a solenoid on the gear handle, also.

One can use a solenoid alike ones used elsewhere in the aircraft, and modify the handle to accomodate it. This would be a rather expensive modification needing power to the solenoid, as well as some kind of connection to the air/ground logic. This could be done by Fokker, alone.

The other suggestion I did would probably be less involving for the mechanical hardware.

It would imply mounting a detector switch operated by the flight control handle or anything moving connected to it, routed to one of the unassigned input pins of the Flight Warning Computer (FWC).

Apart from that, wiring to and modification of the SAP (a new lamp legend on it and because of that a new part number etc...) would probably be mandatory, also.

This would, apart from the hardware, involve software actions to be implemented, also involving the avionics manufacturer:

1. Define a warning in the embedded software of the FWC. This implies defining the input conditions, warning level, prioritisation, flightphase inhibit conditions and "arinc429" output.

2. IIRC the procedures texts themselves reside in the MFDS already, so alike, define the Arinc 429 input, as well the associated procedure text, to be done in the Multifunction Display System firmware.

One way or another, any modification does not come cheap, although, software revisions were probably an ongoing process.

Result: I was awarded 100 Dutch Guilders for the idea.

Below is the letter Fokker provided.


Just a few years later, browsing the Fokker 100 pilots manual, my eye caught a service bulletin in the AOM.

This is service bulletin 48, dated feb 05, 1993, describing a situation where the ailerons were locked during an approach, because of the flt ctl lock being up and locked.

The ability to move the flight control lock in the air apparently proved to be an issue in the real. The bulletin ASSUMED the handle was not properly locked down and may have creeped up during turbulation.

Although an ALIKE Flight Control Lock situation and a single event, depending on the flight phase, it could be just as lethal, but the crew acted upon the situation in time.

Unfortunately, the bulletin does not provide useful detailed information.

Because of copyright, I presume I can not display this page over here.

I could NOT directly crossreference the information of the bulletin to any incident or alike to be found on the internet.


I asked for review of my suggestion again, having the impression there was no progress.

This time, I also sent a copy of the idea to the head of "de vliegdienst" (flight operations), which was the department doing test/ferry/delivery/recovery flights.

Again I got a reward, but even more important was the acknowledgement the problem is real.

According the letter (below) I did get, apparently they started seeking for a solution again.

It also mentioned, apparently TWO events happened already, although it is not known how much this was related to my issue....

(btw: the "lever 3 alert" is a typo in the letter, it should be read as "level 3 alert")


A few years before the Fokker bankrupcy, I "went with the equipment" to a new training center, called Friendship Simulation Company, which was aquired by CAE, later

I quit during 2007, three Fokker 100/Fokker 70 simulators in-house were present in the Hoofddorp building during those days.

I was never aware of any related simulator modification requests, concerning the issue, although I could have missed out on it.

It would be interesting to know, if a solution was found and implemented, despite of Fokker unfortunately went bankrupt.

I presume the situation is still the same, today...

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