This post will document the alterations that have been made to enable the stab trim wheels to utilize four speeds. The post will also highlight several problems encountered during the conversion and document their solution. In addition, the post discusses possible reasons for the erratic behavior of the stab trim indicator tabs.
LEFT: Captain-side stab trim wheel with manual trim handle extended. The white line on the trim wheel is an aid to indicate that the trim wheels are rotating (click to enlarge).
In the previous throttle unit, the power to rotate the trim wheels was from a inexpensive 12 Volt pump motor. The forward and aft rotation speed of the stab trim wheels was controlled by an I/O card. The system worked well, but the single speed was far from realistic.
The upgrade to the throttle quadrant enables the stab trim wheels to rotate at four speeds which are identical to the speeds observed in a Boeing aircraft. The speed is controlled by three adjustable speed controller cards, five relays and a Phidget 0/0/8 interface card – all of which are mounted within the Throttle Interface Module (TIM).
To generate the torque required to rotate the trim wheels at varying speeds, the pump motor was replaced with a encoder capable 12 volt dual polarity brush motor. The replacement motor is mounted on a customized bracket attached to the inside frame of the throttle unit. This style of motor is often used in the robotics industry.
Boeing Rotation Speed
The speed at which the trim wheels rotate is identical to the Boeing specification for the NG series airframe. Simply written, it is:
(i) Manual trim - speed without flaps (slow speed);
(ii) Manual trim - speed with flaps extended (very fast speed);
(iii) Autopilot trim - speed without flaps extended (very slow speed); and,
(iv) Autopilot trim - speed with flaps extended (faster speed than iii but not as fast as ii).
To determine the correct number of revolutions, each trim wheel cycle was measured using an electronic tachometer. Electronic tachometers are often used in the automobile industry to time an engine by measuring the number of revolutions made by the flywheel.
It is important to understand that it is not the rotation speed of the trim wheels which is important, but more the speed at which the aircraft is trimmed. With flaps extended, the time taken to trim the aircraft is much quicker than the time taken if the flaps were retracted.
There is definitely a noticeable difference between the speed that the trim wheels rotate at their slowest speed and fastest speed; however, the difference is subtle when comparing the intermediate speeds.
LEFT: Electric stab trim switch on Captain-side yoke. Whenever the trim is engaged the stab trim wheels will rotate with a corresponding movement in the stab trim indicator tabs (click to enlarge).
Design and Perils of Stab Trim
If you speak to any real-world pilot that flies Boeing style aircraft, they all agree upon a dislike for the spinning of the trim wheels. The wheels as they rotate are noisy, are a distraction, and in some instances can be quite dangerous, especially if your hand is resting on the wheel and the trim is engaged automatically by the autopilot. This is not to mention the side handle used to manually rotate the trim wheels, which if left extended, can easily damage your knee, during an automatic trimming operation.
If you look at the Airbus which is the primary rival of Boeing, the trim wheels pale by comparison; they are quiet, rotate less often, and are in no way obtrusive. So why is this case?
Boeing when they deigned the classic and NG series aircraft did not design the throttle unit anew. Rather, they elected to build upon existing technology which had changed little since the introduction of the Boeing 707. This saved the company considerable expense.
Airbus, on the other hand, designed their throttle system from the ground-up and incorporated smaller and less obtrusive trim wheels from the onset.
Interestingly, Boeing in their design of the Dreamliner have revamped the design of the stab trim wheels and the new design incorporates smaller, quieter and less obtrusive trim wheels than in the earlier Boeing airframes – no doubt the use of automated and computer controlled systems has removed the need for such a loud and visually orientated system.
Problems Encountered (Teething Issues)
Three problems were encountered when the trim wheels were converted to use four speeds. They were:
(i) Excessive vibration when the trim wheels rotate at the fastest speed;
(ii) Inconsistency with two of the speeds caused when CMD A/B is engaged; and,
(iii) Fluttering (spiking) of the stab trim indicator tabs when the electric stab trim switch was engaged in the down position.
Point (i) is discussed immediately below while points (ii) and (iii), which are interrelated, have been discussed together.
(i) Excessive vibration
When the trim wheels rotate at their highest speed there is considerable vibration generated, which causes the throttle quadrant to shake slightly on its mounts.
One of the reasons for the excessive vibration becomes obvious when you compare the mounting points for the throttle quadrant in a homemade simulator to those found in a real aircraft – the later has several solid attachment points between the throttle unit, the center pedestal, the main instrument panel (CDU Bay), and the rigid floor of the flight deck.
In a simulator, replicating these attachment points can be difficult. Also, the throttle is a relatively high yet narrow structure and any vibration will be exacerbated higher in the structure.
LEFT: Example of a stab trim wheel cog and mechanism (before cleaned) from the First Officer side. The picture shows some of the internal parts that move (and vibrate) when the trim wheels rotate at very high speeds. The high and narrow shape of the throttle unit is easily noted (click to enlarge).
Another reason for the cause of the vibrations is the material used to produce the center pedestal. In the classic airframe the material used was aluminum; however, in the NG carbon fiber is used, which is far more flexible than aluminum. Any vibration caused by the rotation of the trim wheels has a tendency to become amplified as it travels to the less rigid center pedestal and then to the floor of the flight deck.
Solving the vibration issue is uncomplicated – provide stronger, additional, and more secure mounting points for the throttle quadrant and the attached center pedestal, or slow the rotation of the trim wheels to a more acceptable speed. Another option is to replace the platform’s floor with a heavier grade of steel or aluminum. This would enable the throttle quadrant and center pedestal to be attached to the floor structure more securely. However, this would add significant weight to the structure. In my opinion, a heavy steel floor is excessive.
By far the simplest solution, is to reduce the fastest speed at which the trim wheels rotate. The rotation speed can be altered, by the turn of the screwdriver, on one of three speed controller cards mounted within the Throttle Interface Module (TIM).
For those individuals using a full flight deck including a shell, the excessive vibration is probably not going to be an issue as the shell provides additional holding points in which to secure the throttle quadrant, MIP and floor structure.
(ii) Inconsistency with two of the speeds caused when CMA A/B is engaged
When the autopilot (CMD A/B) was selected and engaged on the MCP, the rotation of the trim wheels would rotate at an unacceptable very high speed (similar to run-away trim).
The mechanics of this issue was that when the autopilot was engaged, the electronics was not activating the relay that is responsible for engaging the speed controller card.
(iii) Fluttering of the stab trim indicators
When the electric stab trim switch was depressed to the down position, it was observed that the stab trim indicator tabs would often flutter. Although the fluttering was mechanical and had no bearing on the trim accuracy, or speed at which the aircraft was trimmed, it was visually distracting.
A possible cause for the run-away trim was electromagnetic interference (RF) generated by the high torque of the trim motor. The higher than normal values of RF were being ‘picked up’ by the relay card, which were causing the relay to not activate when the autopilot was engaged. Similarly, the fluttering of the stab trim indicator tabs, was thought to have been caused by RF interfering with the servo motor.
There were several possibilities for RF leakage.
(i) The high torque of the motor was generating and releasing too much RF;
(ii) The wire lumen that accommodates the cabling for the throttle is mounted proximal to the servo motor. If the lumen was leaking RF, then this may have interfered with the operation of the servo motor;
(iii) The servo motor was not digital and did not have an RF shield attached;
(iv) The straight-through cable from the Throttle Communication Module (TCM) to the Throttle Interface Module (TIM) did not have RF interference nodules attached to the cable.
To counter the unwanted RF energy several modifications were made:
(i) Three non-polarized ceramic capacitors were placed across the connections of the trim wheel motor;
(ii) The analogue servo motor was replaced with a higher-end digital servo with an RF shield;
(iii) The straight-through cable between the TIM and TCM was replaced with a cable that included high quality RF nodes; and,
(iv) The wires from the servo motor were re-routed and shielded to ensure they were not lying alongside the wire lumen.
Manual trimming (turning the trim wheels by hand) is not implemented in the throttle quadrant, but a future upgrade may incorporate this feature.
In the earlier conversion, the stab trim cut-out toggle was not functional and the toggle had been programmed to switch off the circuit that powers the rotation of the trim wheels. Having the ability to disconnect the rotation of the trim wheels is paramount when flying at night, as the noisy trim wheels kept family members awake.
LEFT: Stab trim cut out switches with spring-loaded cover open on main and closed on autopilot (click to enlarge).
The new conversion does not incorporate this feature as the trim cut-out toggle is fully functional. Rather, a push-to-engage, green-coloured LED button has been installed to the forward side of the Throttle Interface Module (TIM). The button is connected to a relay, which will either open or close the 12 volt circuit responsible for directing power to the trim motor.
The method used to convert the stab trim indicators has not been altered, with the exception of replacing the analogue servo with a RF protected digital servo (to stop RF interference).
LEFT: Stab trim indicator tabs (Captain side). The throttle is from B737-500. The indicator tabs on the NG airframe are slightly different - they are more slender and pointed (click to enlarge).
To review, a servo motor and a Phidget advanced servo card have been used to enable the stab trim tab indicators to move in synchronization to the revolution and position of the stab trim wheels. The servo card is mounted within the Throttle Interface Module (TIM) and the servo motor is mounted on the Captain-side of the throttle unit adjacent to the trim wheel. There is nothing exceptional about the conversion of the stab trim indicator tabs and the conversion is, more or less, a stock standard.
Is Variable Rotation Speed Important to Simulate
As discussed earlier, it is not the actual rotation of the trim wheels that is important, but more the speed at which the aircraft is trimmed. In other words, the speed at which the trim wheels rotate dictates the time that is taken for the aircraft to be trimmed.
If the trim wheels are rotating slowly, the movement of the stab trim indicator tabs will be slow, and it will take longer for the aircraft to be trimmed. Conversely, if the rotation is faster the stab trim indicator tabs will move faster and the aircraft will be trimmed much more quickly.
Final Call - is Four-speed Trim Worthwhile
Most throttle conversions implement only one speed for the forward and aft rotation of the trim wheels with the conversion being relatively straightforward.
Converting the throttle unit to use four speeds has not been without problems, with the main issue being the excessive vibration caused by the faster rotation speed. Nevertheless, it is only in rare instances, such as when the stab trim is engaged for longer than a few seconds at a time, and at the fastest rotation speed, that the vibration becomes an issue. If the rotation for the fastest speed is reduced, any vibration issues are alleviated – the downside to this being the fastest speed does not replicate the correct Boeing rotation speed.
For enthusiasts wishing to replicate real aircraft systems, there is little excuse for not implementing four-speed trim, however, for the majority of flight deck builders I believe that two-speed trim, is more than adequate.
Below is a short video, which demonstrates the smooth movement of the stab trim indicator tabs from the fully forward to fully aft position. The video is only intended to present the functionality of the unit and is not to represent in-flight settings.
Below is short video that demonstrates two of the four rotation speeds used. In the example, manual trim is has been engaged, beginning with flaps UP, flaps extended, and then flaps UP again. The rotation speed of the trim wheels with flaps extended (in this case to flaps 1) is faster than the rotation speed with flaps UP. The video does not reflect in-flight operations and is only to present the functionality of the unit in question.
Electromagnetic Interference (RF) – RF is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source (see Wikipedia definition).
MCP – Mode Control Panel.
MIP – Main Instrument Panel.
Stab Trim Indicator Tabs – The two metal pointed indicators located on the throttle unit immediately adjacent to the %CG light plate. If not using a workable throttle unit, then these tabs maybe located in the lower EICAS as a custom user option.
Servo Motor – Refers to the motor that powers the stab trim indicator tabs.
Trim Motor – Refers to the motor that powers the stab trim wheels.