GOLF' Pilot Aviation BLOG: The main cause of accident on Pilatus PC6: HORIZONTAL STABILIZER TRIM

During every training with the PC6 Pilatus there is a deep discussion about the horizontal trim set up and how it works. PC6 looks a simple machine ma if you don't follow few simple rules basically it's a fathal machine for the pilots and occupants on board. For sure I could say it's an airplane which requires a solid basic training (and not few hours to be safe).

One of the main reasons of PC6 accidents is the result of loss of control of the aircraft following a takeoff with an incorrect horizontal stabilizer trim setting. Normally there are some contributory factors like the incorrect loading of the aircraft, the difficulty of re-trimming the horizontal stabilizer rapidly and an inadequate standard of aircraft operation.

PILATUS urges all pilots to adhere to the procedures given in the AFM.Special attention must be paid to correct trim settings prior to take-off.

During my initial training on PC6 B2H2 in France the basic handling was centered about the use of manual trim. You had to be familiar about leverages over your shoulders, one for the trim and one for the flap. French instructors used to say to me "PC6 is a kinda of truck" (Pilatus est un tracteur...), the engine is so powerful that generates an airflow (and a propeller wash) so strong that is very difficult to contrast manually with a handle crank. This means that you should reduce throttle and then move the trim. It's very difficult when you have the nose high to reduce engine, you should make some turns with the handle crank and then give back throttle very soon. The worst scenario is when you have also a wrong flaps setting. Flaps down increase the trend of the pitch up, normally the bigger handle crank is for flap and the smaller one for trim. If after take off you notice a wrong trim and move the wrong handle crank by mistake you are almost dead with an even more increased attitude up to the stall.

This the Tail of F-GOME (B2H2) during my preflight inspection

Therefore the classic aircraft has a manually controlled elevator operated by cables and push- pull rods from the control column. As forementioned the Pitch trim is achieved by varying the incidence of the horizontal stabiliser through the rotation of a crank positioned on the cockpit roof on the centreline of the aircraft.

This crank has a range of 19% turns (clockwise as viewed by the pilot) from the full nose-down trim position to the full nose-up position. Aim position indicator adjacent to the handle is calibrated linearly from 7 divisions nose up, through zero, to 5 divisions nose-down. So I had to keep in mind all the times how many turns to do.

Climbing and stalling speeds

The following data is extracted from the aircraft's Fight Mamual, as applicable to a gross weight of 4,850 bs (2,200 kg)

Recommended rormal climb speed (takeoff power, flaon 28)........................... 56 knots IAS
Recommended climb speod for maximun perfomance (lake-off power, laps 28) ...45knots IAS
Note: The manufacurer states that the aircraft must be flown accurately if the aircraft is climbed at the maximum performance climb speed.
Stalling speed (power off, faps 28, wings level).................................... 45 knots IAS
Note: The stalling spoed with power on is not given, but will be a lower value.

from my personal notes

But why is so difficult to trim the Porter manually with high power? Basically with the stick we move the elevetors, but with the trim we move all the horizontal tail (like a stabilator). The gimbal of the tail is in front, the action of the trim is applied behind. If I push down the stick the elevetor goes up and I have to win this force against if I move the trim (which moves the tail in opposite direction). For this reason I have to realase the pressure from the stick, move the handle crank in the right direction, and then apply pressure on the stick again. Not easy while you are under upset recovery manouver. Another way to win this force si to reduce throttle and so the propeller flow (counter instict if your nose is up).

A tecnique taught by my instructor in France is to put the handle crank at 90 degrees position above your shoulder, and then move it towards to you for trim up, and opposite direction to trim down. It really works!

The Trims control is the main check before take off. It's part of the 5 items checklist before take off (See this post in Italian).

In 2000 PILATUS went one step further and developed a Trim Warning System, which is offered thru Service Bulletin 180. This system will warn the pilot already on the ground about a dangerous out of trim situation.
The installation of a trim warning system such as SB 180 was mandated by FOCA, DGAC and ENAC and PILATUS strongly urges all operators to install the trim warning system as offered thru SB 180.

A modern touch about the last PC6 is the fully electrically driven trim system for all three primary flight controls. There is no manual hand crank system now. The rudder trim takes a little getting used to as there’s almost no feel or no kick back through the pedals when you’re in balance but you get used to it.

The pitch trim is the most important of the three trims. The hat switch on the control stick controls an electrical motor that moves the entire horizontal stabiliser which is a VERY powerful control surface. So much so that if you get a trim runaway (where a switch gets stuck and the trim motor keeps going until it reaches it’s control limit), you will struggle to maintain control of the aircraft.

In order to save you getting into a situation where the trim is locked against a limit, Pilatus have installed a trim interrupt switch right next to the power levers which when engaged will disable all trim switches. Once engaged you then pull the circuit breaker of the problem trim motor and close the interrupt switch. The elevator trim also has a secondary motor which can be engaged to re-trim the stabiliser should the primary one have failed as it is such an important control surface.

The Trim Runway Emergency was part of my B2H4 differential training

The horizontal stabilizer electric trim system consists of:

A dual motor (Main and Alternate Motors) electrically-operated linear actuator, one end attached to a support frame in the rear fuselage and the other end attached to the horizontal stabilizer.
A normal trim control system which incorporates two three-position spring-loaded trim switches and two relays. One switch is located on each of the two control column grips. These switches control the Main actuator, which operates at twice the speed of the Alternate Trim System actuator motor.
An Alternate Trim Control System which incorporates an actuator control switch positioned on the instrument panel and labelled ALTERNATE STAB TRIM, NOSE DN, NOSE UP.

An Interrupt system which incorporates a guarded switch positioned on the instrument panel shelf and labelled TRIM INTERRUPT. When the TRIM INTERRUPT switch is in the ‘NORMAL’ position, both the Main and Alternate systems are active. When the TRIM INTERRUPT switch is in the ‘INTERRUPT’ position, both the Main and Alternate systems are inactive.

An electrically-operated trim position indicator located on the upper left side of the instrument panel.

Main System

Normal longitudinal trim operation is accomplished by operating either the trim control switch on the pilot's control column grip, or, the trim control switch on the co-pilot's control column grip. Both switches are similar and are spring-loaded to neutral. The trim control switch on the pilot's control column has priority. Nose DOWN (DN) trim is accomplished by moving the switch to the up position and Nose UP trim is accomplished by moving the switch to the down position. Operating either trim switch energizes the actuator which moves the horizontal stabilizer.

In NORMAL operation the stabilizer trim actuator is powered via a circuit breaker marked STAB TRIM.

Alternate System

If the normal trim control system fails, an alternate trim system is available. The alternate trim system consists of a switch labelled ALTERNATE STAB TRIM, NOSE DN, NOSE UP.

An electric trim indicator, and a trim warning light were installed in the upper left portion of the instrument panel. The light will illuminate if "full-up" trim is set, and the engine is producing over 80 percent power. A placard stating, "Set Correct Trim for Takeoff", was installed on the lower instrument panel in front of the pilot position. The airplane's flight manual contains a "Before Takeoff" warning, which states, in part: "Warning - An extreme out-of-trim stabilizer can, in combination with loading, flaps position and power influence, result in an uncontrollable aircraft after the aircraft leaves the ground." In addition, a caution states, in part: "Caution - Failure to set correct trim settings will result in large control forces and/or unrequested pitching/yawing." Pilot actions listed in the "Before Takeoff" checklist include stabilizer trim settings.
The pilot's incorrect setting of the stabilizer trim and his failure to maintain adequate airspeed during takeoff initial climb, usually results in a stall. A factor contributing to the accidents is an inadvertent stall.

from my personal notes

During my differential training one of the main point of the syllabus was the emengercy of trim Runway. Normally we used to perform it durind descend after dropped the jumpers in Sardinia.

Trim Runway Emergency

The most difficult part, after identified the trim runway, is to pull up the breaker during an upset situation, especially because the position is not very reachable. The cheklist could seem very long, but the principle is to deactivate the trim motor (which probably is pushing or pulling continuosly the elevetor) and then use the alternate trim. Initially you have to identify which trim has the malfunction (main or alternate), that's why you pull and push the STAB Trim Circuit Breaker. The main trim is stronger than the alternate trim.