We are trained to test certain systems at every preflight inspection to give some level of confidence they’ll be working when needed. We switch on the battery master, then pull up on the lift detector out on the left wing and listen for the solid warning horn. We also push the gear down light socket in just a bit, but not far enough to engage the test light, so we can hear the intermittent sound of the gear warning. At least that’s what we are trained to do. This is all fine and good, of course, but do we know if they work in the situation for which they’re intended?

One would think we pilots have no memory skills at all considering all the reminders we are given. There are placards all over the instrument panels reminding pilots to do everything from turning off strobe lights in the clouds to turning on the electric fuel pump if the engine quits. The electronics of today flash a reminder, in plain text, to change fuel tanks every thirty minutes or tell us a door is open or that some magic box isn’t communicating with some other magic box. It seems odd that the two most commonly needed and frequently ignored warning systems just blow a horn at the pilot no labeling at all.

Testing, testing, 1, 2, 3, …

When ground testing the stall warning or gear warning during preflight, one should consider what  that test is actually doing. Let’s explore these two fairly simple warning systems and find out what value the ground test actually yields. We’ll use an R182 as our subject, as it’s a very good generic representation of all the Cessna piston singles.

Stall warning systems are straight forward in their design. Tracing through the series components starting with the electrical buss, the system consists of a circuit breaker, dual warning assembly, lift detector (stall switch), and the speaker. Some aircraft have the speaker built into the dual warning assembly while others actually use the aircraft cabin speaker.

The R182 is one that uses the cabin speaker which is a special part designed to operate with two independent audio inputs. As one might guess, that speaker is no longer available from Cessna, and I have yet to discover an easy direct replacement unit. Read this as a note to take good care of this speaker.

When the lever on the lift detector moves up, it closes the normally open contacts of the internal switch. This applies a ground path to the warning unit which activates the audible stall warning. This simple test lets you know that all the components are working but tells you nothing about whether it will engage in flight when it’s supposed to.

Lift detector switch viewed from inside the wing.

Lift detector switch viewed from inside the wing.

The gear warning is a little more complicated and has more variations between aircraft models. It shares the same circuit breaker and dual warning unit as the stall system, hence the term “dual”. The gear warning is still activated by a switched ground, but the source is a switch somewhere on the throttle control or a flap position switch or both. There is also an input to the dual warning unit to tell it when the green down and locked light is illuminated. Note that I didn’t say it knows when the gear is down, just that the green light is illuminated.

The gear warning ground test is to push the green down and locked light socket in toward the lamp test position but not quite far enough to actually test the lamp. Any time there is power to illuminate the green light, that same power is sent to the dual warning unit to inhibit the gear warning circuit. Putting the socket in a position halfway between getting power from the down lock switches and the lamp test power removes any power to the lamp and thus the warning inhibit line to the dual warning unit. With no power at the green light gear down input, the warning unit looks to the throttle and flap switch inputs to see if the plane is in a landing configuration. If it is, then the warning sounds.

Some added testing can be accomplished by pushing the throttle forward to ensure the warning is inhibited. Raising the flaps should also inhibit the warning. If your aircraft includes a flap switch, then both the throttle and the flaps must be in a nonlanding configuration to inhibit the warning horn.

Real World

We recently had a long time R182 customer arrive with a nonfunctioning gear warning system that was discovered on the flight to the shop. The plane had recently undergone a major avionics upgrade and was heading for the paint and  interior shop after the annual. This annual was part of getting the plane in tip top shape all the way around.

We repaired a broken wire at the throttle switch during the previous annual and assumed something had run amuck with that repair. A quick check revealed the throttle switch and associated wiring was intact and working. A discrepancy was entered in the work order to do more thorough troubleshooting after the annual inspection phase was completed.

During the annual inspection it was also discovered the stall warning did not work. Now we had two failures with common components which altered the troubleshooting expectations significantly. Our efforts were quickly focused on the dual warning unit and the speaker, both common to the stall and gear warnings. The speaker worked well as an audio generator for the avionics, so we decided to pursue troubleshooting the dual warning unit first.

We accessed the dual warning unit connector to check for proper inputs and outputs. We found good power from the circuit breaker at pin 1, a good ground at the case, and the switched grounds from the flaps, throttle and lift detector were all working on command. The switched power from the green gear down light was operating as requested as well. The only item remaining to test was the speaker output at pin 6.

Dual warning unit conveniently located behind the headliner just above the pilot's door

Dual warning unit conveniently located behind the headliner just above the pilot’s door

We could have used jumper wires from the connector to a test speaker or an oscilloscope to listen or look for an audio signal. No spare speaker or oscilloscope was handy, so we just pulled the ship’s speaker down to check the connections. This special speaker is distinguished by having four terminals instead of the normal two. One terminal is for avionics speaker audio, another for stall/gear warning audio, and the other two are for ground returns.

Once the speaker was down and in view, we noticed there were only two wires attached: one blue and one green. Green is the old school standard color to denote a speaker wire. Blue is often used for head phones audio but in this case is the color Cessna used for the speaker wire from the dual warning unit. The two missing ground wires were a likely problem but turned out only partly to blame.

As we traced the blue and green wires a few inches upstream, we saw they were spliced into a very new shielded twisted pair wire. This was definitely not original Cessna and had all the evidence of being newly installed with the avionics upgrade. The rest of the story was then obvious.

When connecting the new audio system to the old airframe, the shop apparently decided to continue using the original speaker. It was in terrible condition with some of the cone material torn and even a bit missing. Someone did make an attempt to repair it with some tape, avionics approved quality I’m sure. With today’s high tech headsets, speakers are only rarely used. With that in mind, the shop may have determined it would be fine to leave the old speaker in place and save the trouble of finding a new one.

The original speaker, complete with tape repair. Note the four connections near the center cone for the dual application.

The original speaker, complete with tape repair. Note the four connections near the center cone for the dual application.

Instead of just soldering the new wires directly to the old speaker, the shop used a few inches of the original green and blue wires and spliced the new wires to the remains of the old ones. The new shielded pair wire provided a proper speaker ground return to the audio panel which replaced one of the two original black speaker ground wires. The other ground wire was discarded. All the avionics audio tests showed normal operation of the speaker.

The avionics shop was apparently unaware of the importance of the little blue wire and didn’t take the time to investigate its function. It must have been assumed it served no useful purpose. Fortunately, they did leave the old speaker in place which allowed us to return it to the correct configuration. We left the two avionics audio wires as the shop installed them, connected the blue wire from the dual warning unit and added the missing ground wire. Suddenly, we had good stall warning and gear warning horn operation restored.

The new speaker wire spliced into the old blue and green wires with original connectors at the end.

The new speaker wire spliced into the old blue and green wires with original connectors at the end.

Lessons Learned

Ground testing is a must but is no substitute for finding out if the systems work as advertised in flight situations. The next time you’re out flying for fun, it’s a good idea to find out if the stall warning is engaging at the appropriate speed. Please follow all the normal precautions for stalls. It’s a great idea to take along a flight instructor for some refresher work while you’re at it.

I make it a general practice to cause the gear warning to sound on every flight. I then know what conditions engage the system and reinforce the specific sound with the landing gear. Your specific POH should tell you within what parameters the gear warning operates. It doesn’t require you to do any dangerous maneuvers or put anything at risk, just the normal approach to landing with a short throttle pull on downwind to determine when the warning sounds.

Ground testing of these two nonplacarded and low tech system warnings should be performed at every preflight inspection. The checks are simple and require only seconds of effort. It’s a terrible thing to discover either one of these warnings not working at the very time they are needed most. A stall warning that operates at too low an airspeed (too high angle of attack) or a gear warning that comes on after the plane is on its belly isn’t much help. A properly working gear warning can save a tremendous amount of hassle, and the stall warning can save your life.

Copyright © Paul New 2014. All rights reserved.