Unintended accelleration and When Pitch Trim Systems Turn Ugly

When Pitch Trim Systems Turn Ugly.

Indeed, they can get pretty nasty. I remember my boss years ago telling me, it took all his strength, plus those of a mechanic sitting next to him to keep the plane under control, when trim runaway occurred. Thankfully, they were able to get the plane stabilized, and then troubleshoot the problem and return to the ground safely. In a single pilot op, apart from the strength issue, not sure it would have even been possible.

That being said, pitch trim runaway has plagued airplanes for years, whether it be due to control failure, wire harness fatique, or relay lockups, and those were the simple ones…. As we evolve towards more and more fadac aircraft, and push greater and greater tech into the cockpit, concerns of such will likely increase, but also engineering is a lot more diligent and forward looking than the days of the past, where such systems were more electromechanical in nature.

Along that line of thought, is unintended acceleration in passenger cars. Certainly with the advent of cruise control, the potential for such has existed for some time. Although, granted, the old analog cruise controls of days long since past were pretty robust, and when they failed. the catastrophic mode was a statistical improbability. Most of the time, the system would just go dead, or worst case lock up…. not go intermittent as far as accelleration goes.

With todays engine idle control, emissions control, and even things like idle boost for purposes of power steering or ac acutation, the engine control computer, and throttle positioner servos, and play a much more critical role, as well as often function in more of a real time mode, and unlike kick up solenoids, are not mechanically limited to only small changes in rpm.

This does post a bit of a concern in two areas. First, from a EMC point of view, only a few models get tested, and it is hoped and assumed, that car A is a representative sample of a wide range of bodies using somewhat equivalent electronics and systems. Should there be an oddity, like the wiring harness mounts on the mid year 91 Ford Ranger which results in a huge susceptibility hold, that could come back to bite in a huge way. In 91, it was a no start issue, and thus safety was not impacted… however, what if an immuity hold pops up, and causes a fault in the throttle servo subsection. That could potentially be a life critical problem… and even more so, what happens as vehicles age…. harnesses droop, insulation degrades, and grounding potentials could end up all over the place, especially in the frequency domain.

Secondly is the issue of software, of which the vehile owner typically has little control, The only thing they really can do, is make sure their vehicle is taken in for scheduled maintenance, and especially so if they get a notice from the manufacturer. Almost all vehicles have flash memory, and thus it is possible for the car makers to update software long after a vehicle has left the manufacturing floor. Thus, while it may seem spendy to bring a car in for seemingly trivial maintenance such as oil, fluids, and a quick inspection… this is also where new firmware may be uploaded into the vehicles computer, and it a new release is available, and it catches some bugs, it could well be a life saver.

Thus, with EMC issues as a vehicle ages, as well as firmware issues, sooner or later it is likely more and more cases of unintended accelleration will occur. Fortunately, there is the brake solenoid interlock which prevents moving a gear selector unless the brake is on, and likely this will not only prevent user error as it has shown to do, but also serve as a backup safety to prevent unintended accelleration resulting in bodily injury or property damage. Two things are key…

1, the interlock should never be removed, sure its a spendy part, and prone to wear and early failure, but it indeed is a mission critical device.

2. When shifting from park to drive, ones foot should be firmly on the brake… not just toe pressure to actuate the interlock system. I caught myself doing that the other day, which is what led to a little digging, and resulted in this blog article.

Its not unlike the days when I used to really enjoy the yoke controlled electric trim… not only for inflight, but for pretty much all modes, that was until a buddy had a Dutchess freak out on him, and that brought to mind my bosses experience too. After that, I only used it in cruise, preferring instead to keep the system off below 1000 ft. Electronics in the cockpit and in the car are great convenience items, but they should never stand in the way to where safety is compromised.