Flap Stories

Flap Stories

I’d like to tell you a few interesting things that I have experienced over the years involving flaps. I hope you will find them interesting and educational.

Let me begin by reviewing the basic flap system design in King Airs and, with minor changes, in most other Beechcraft airplanes. The semi-fowler flaps – ones that extend aft to increase the wing’s chord as they go down – are driven by jackscrews and ride on tracks connected to the wing’s rear spar structure … two tracks and one jackscrew per flap segment or flap panel. As my colleague, Dean Benedict, has written in this magazine, there are rollers, bushings and Teflon washers that connect each segment to its two tracks and it is unfortunately common to find these installed incorrectly, leading to track damage that can be time-consuming and expensive to repair. The jackscrews are driven by flex-drive cables that pass into the fuselage where they connect to a transmission or gearbox assembly that is mounted under the cabin aisle floorboards on the forward side of the rear spar.

Mounted beneath the gearbox is the drive motor – a 28-volt DC, reversing, electric motor. In this case, “reversing” means that it can run in two directions for up and down flap travel, depending on which of its dual field windings is energized. When the motor is running in the “up” direction, the “down” winding is acting as a generator, and vice versa. However, with no demand, no “load,” placed on that generator it is providing insignificant resistance to motor rotation. Unlike in Bonanzas and other models with a single flap panel per side, the two panels per side on the King Air require more power to operate and, in turn, there is more momentum to keep them coasting after the motor is no longer receiving power. This coasting momentum can drive all flap panels to the absolute limit of track travel, putting undesirable strain on the components. To prevent this coasting travel, Beech uses a Dynamic Brake Relay. It does the following: Whenever power is removed from one field winding, the other winding is now shorted to the airframe. This puts an “infinite” load on the winding that is acting as a generator. With huge resistance to rotation being provided by that winding and no longer any driving force being received by the other winding, the assembly comes to a screeching halt. It acts as if some strong mechanical brake were suddenly applied to the motor’s output shaft but it is all done by a magnetic field, not by a physical brake. Cool!

The flap motor, gearbox, drive cables and Dynamic Brake Relay.

The Dynamic Brake Relay operates whenever the flaps hit a Limit Switch – Up, Down or Approach. It also activates in earlier King Airs – the ones in which the flaps may be stopped in any position between Approach and Down – when the flap handle is moved from Down to Approach while the flaps are extending between Approach and Down. The fact that the flaps will stop immediately means that when we want to put them at 60%, we can wait to move the handle from Down to Approach until we see the indicator pointing right at 60%. They won’t coast on down to 65% or more, not with our Dynamic Brake friend!

What if they do coast a bit? If this is happening, you will likely also find no free play on the flaps when you move their trailing edges up and down on the preflight inspection since they have coasted to the ends of the tracks. The likely cause of this is a bad Dynamic Brake Relay or highly worn flap motor brushes.

Back to the motor/gearbox connection: The output shaft of the motor acts as the “worm” that rotates two shafts, “worm gears,” one rotating clockwise and the other counterclockwise, or vice versa, depending on whether Up or Down is selected. Both left and right outboard flap segments are connected to one of these shafts and the inboard segments are connected to the other shaft. Although extremely rare, if one of these shafts experiences a stripped gear such that the motor cannot drive it, then we would lose both left and right inboard or both left and right outboard flap segments … it would never present us with an asymmetrical situation.

Inside of the flap gearbox or transmission showing worm and worm gears.

In 1975, a colleague of mine was delivering a factory new E90 to Beechcraft West, a Beech distributorship in Southern California, located at Van Nuys (KVNY). In those “good old days” no flight restrictions existed over the Grand Canyon so on their descent into Las Vegas for a refueling stop, they were enjoying the canyon views while descending near redline speeds over the Colorado River. As the passengers “oohed and aahed” their way in the descent while the pilot S-turned over the canyon, my friend suddenly felt the airplane balloon upward. “Oops, I think I just touched something!” said the passenger sitting in the co-pilot’s seat. It didn’t take long to realize that the passenger had accidently moved the flap handle from Up to Approach as he was leaning over to look out of the left-side cockpit windows. (This incident led to Beech adding the little “wall” on the right side of the flap handle to decrease the chance that it could be moved accidentally.) The pilot retracted the flaps and continued to Las Vegas. He used 100% flaps for landing and all was well. But as he taxied to the FBO, Ground Control said, “Hey, King Air, your inboard flaps are up, but your outboard flaps are still down. You aware of that?”

Until then, he was not aware of any problem. When the flaps were accidently sent to the Approach position at a speed well over the limit, apparently the teeth on one of the transmission’s worm gears had been overloaded so that they were weakened enough such that they failed completely in another couple of cycles.

For split flaps to occur – by this we mean one segment being out of sync with the three others – we must have a jackscrew failure: Either the jackscrew itself is faulty or the jackscrew is not being driven by its drive cable: the cable broke internally or it became disconnected from the jackscrew or the transmission.

Twice in my flying hours I have personally experienced split flaps. Once was in an A90 – that I talk about in The King Air Book – and the other time was in a Duke … that has a single flap segment per side, unlike the King Air. In both cases I was very pleasantly surprised to find that the outcome was basically a nonevent. Further analysis suggests that this is not too surprising since the lift/lack-of-lift this malfunction causes has an asymmetrical force acting on the wing’s inboard area whereas the aileron and their trim tabs apply force on the outboard area. Inboard: Less rolling moment arm; Outboard: More rolling moment arm.

The Split Flap Protection switch and its connections to both inboard and outboard flap segments.

Compared to the King Air 90-and 100-series, the 50-inch wider wing center section of the 200- and 300-series means that flap asymmetry will contribute more roll force than before. Therefore, these later models incorporate a Split Flap Protection system. As we have discussed, a drive shaft failure that renders both inboard or both outboard segments inoperative simultaneously leads to no roll tendency. It is only when one segment stops and the other three continue working that asymmetry occurs.

The Split Flap Protection system works in this manner: The Flap Control electrical circuit – the same circuit that includes the switches that the flap handle actuates and the limit switches – includes two additional switches. They are connected to the leading edges of the flaps, one on each side, between the inboard and outboard flap segments. (If the flaps are inspected while they are down, the switches are rather easily located and seen.)

As long as the two – inboard and outboard – flap’s leading edges remain closely side-by-side, as they should, then that side’s split flap switch remains closed. But if one segment fails to move while its neighbor moves correctly, the leading edges are no longer side-by-side and the switch opens. As soon as this occurs, the absence of control circuit power causes the Dynamic Brake Relay to instantaneously stop the motor and the still-operating segments. The POH tells us that the stopping action will take place by the time the flaps are 3°- 6° out of sync. Since full flap extension is 35°, it means we never exceed a 20% differential.

How do we handle this split flap condition? We cannot fix the problem. Until maintenance takes place, we are stuck with the flap setting that we now have. The obvious procedural change we need to address is to determine and use a new proper VREF landing speed. Since the flap position indicator gets its information from the right inboard flap segment only, realize that its position may not be the best measure of the overall average extension. “Eyeball” all of the segments, then make an educated guess of the proper speed, somewhere between the Full flap and No flap VREFs.

Before continuing, I need to clarify a minor item. The two split flap protections that I have been discussing did not become actual microswitches until 1979. All of the 300-series use the switches since they appeared later. But the 200-series, prior to serial number BB-425, use a surprisingly odd design. A standard 5-amp cylindrical fuse is mounted within a fuse holder, one with the springy metal clips that hold the fuse. A clamp connects a short wire to the fuse. When the 3°-6° separation occurs, the wire yanks the fuse out of the holding clips, shutting down the circuit. Who came up with that plan?! As funky as it seems, however, it works quite well!

How often do King Airs have split flaps? Almost never. Perhaps of more interest is how often does the Split Flap Protection system malfunction and leave us with inoperative flaps even though the flaps themselves are fine? I have been pleasantly surprised to find that this additional safety feature has proven to be almost 100% trouble-free. Nice!

In 1985 I had the pleasure of flying with one of my recurrent training customers to attend the Paris Air Show. His model 200 was one of the first to be modified with all of the various Raisbeck Engineering King Air STCs that existed at that time. We met James Raisbeck in Europe and used the airplane, prior to arriving in Paris, for some demonstration flights. A number of these flights were in Norway. Landing on some relatively short strips perched on the walls of fjords was lots of fun and certainly showed the airplane to be exceedingly capable. Upon landing for an overnight stay in Bergen our flaps did not retract.

No CBs were popped and no burned-up-flap-motor odor had been detected. I wiggled the Up limit switch and it seemed normal. Maybe split flap protection problem? In our travel kit we had a small jumper wire with alligator clips on both ends. I jumpered around the left switch – easily accessible since the flaps were down – and tried retraction again: Nothing. Moved to the right side: Success! I took off one of the Control circuit wires going to the switch and screwed it on to the other switch terminal, completing a circuit that bypassed the switch completely. Legal? Of course not. A work-around to complete the mission? Yes. I will also add this: For the rest of that entire series of flights, until we safely arrived back in the States, I was always occupying a cockpit seat and watching the flaps carefully whenever they operated. Had any roll tendency developed I would have been a rather fast-acting human split flap protector! When I bid farewell to the owner-pilot I emphasized that now he should have his shop find and fix the actual switch problem. It was time to retire the “temporary” work-around.

A little over a year later I was pre-flighting this same airplane during a recurrent training session. You guessed it: The switch was still bypassed. We did not fly until it got fixed. In another episode that I included in The King Air Book, I came across a 200 that was totally missing split flap protection switches on both sides. They were simply not there and the wires were screwed together to bypass them. By the next day they had been “found” and reinstalled, so we finally flew … a head-scratcher for sure.

One of my more recent articles, dealing with descent planning, was entitled “Just Because You Can, Doesn’t Mean You Should.” I will close this month by applying that same principle to flap operation … landing gear, too. As you know, there are maximum speed limits for flap and gear operation. When you get behind in your descent and approach planning and/or ATC is not making it easy for you, that’s when you are justified in extending the flaps and/or gear right at the maximum speed limit. But normally? With proper planning and execution? There is no reason for continually utilizing the limits. Your equipment will be subjected to a much easier life if we include a 20 or even 30-knot buffer, delaying extension until we are well below the limits. It might even save a few maintenance dollars!

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