The pilot of an airplane with retractable landing gear reports that the gear warning horn sounds on every approach when the power is pulled back, even with three green gear lights showing. The last shop replaced the gear warning unit, the box that decides when to sound the horn. The horn still sounds.
The warning unit is built from logic. If you can read its logic, you can tell whether the box is bad or one of its inputs is wrong before another box is bought.
Three green lights, power back, and the horn sounds.
Term
Gear warning horn
A horn in the cockpit that sounds when the throttle is retarded (pulled back) and the landing gear is not down and locked.
Term
Digital logic
Circuits that work with only two conditions, on or off, and decide an output from their inputs.
When the horn sounds: FAA-H-8083-31B, pages 10-74 and 13-23.
Learning Objectives
When you finish this lesson you can do the four things below. The check at the end asks about each of them.
1Statethe two logic states, the names used for each, and why a reading in the gap between the bands is a fault.
2Findthe output of an AND, OR or NOT gate for any set of inputs.
3Buildthe truth table for a two-input gate and for a NOT gate feeding an AND gate.
4Decidefrom a gate’s inputs, whether a warning fault is a bad input or a bad gate.
The three gate symbols you will read on wiring diagrams: AND, OR and NOT.
Term
Logic gate
A circuit with one output whose state is set by its inputs, by a fixed rule.
Term
Truth table
A table that lists the output for every possible combination of inputs.
Lecture Video
The Transistor Switch
In the Transistors and Inverters lesson you used a transistor as a switch. No base current turned it fully off, called cutoff. Enough base current turned it fully on, called saturation. Logic gates are built from transistors used exactly this way. Answer from memory.
The same transistor switch, cut off and saturated.
From Transistors and Inverters
In a switching circuit, how many useful states does the transistor have?
Source: FAA-H-8083-31B, page 11-25.
Logic 1 and Logic 0
Digital logic recognizes exactly two states. There is no third state and nothing in between. One state is called Logic 1 and the other Logic 0. In a circuit the two states are two voltages: usually voltage present for Logic 1 and no voltage for Logic 0. Manufacturers write different words for the same two states.
Digital Building Blocks
Select a marked passage, or select Next, to read what it means.
There are two conditions that may exist, 1 or 0. In a digital circuit, these are equivalent to voltage or no voltage. Within the binary system, these two conditions are called Logic 1 and Logic 0. Using just these two conditions, gates can be constructed to manipulate information.
4 marked passages
Marked passages
Select a marked passage in the text, or select Next.
Source: FAA-H-8083-31B, page 11-25.
Logic 1 = high = true. Logic 0 = low = false.
Names for the Two States
Put each name with the state it means. Drag the card, or tap a bucket. Positive logic means the more positive voltage is Logic 1.
Logic 1
The high state
Tap the bucket where the card belongs. On a computer you can also drag the card.
A logic state is a band of voltage, not one exact voltage. A logic family, a group of logic circuits built to the same electrical rules, defines a range that counts as low and a separate range that counts as high, with a deliberate gap between them. A working signal passes through the gap only while it switches. A receiver that sees a voltage parked in the gap has no defined state.
Low
Gap
High
High band 2.0 V to 5.0 V. Logic 1.
Gap 0.8 V to 2.0 V. Not a state. A working signal only passes through.
Low band 0 V to 0.8 V. Logic 0.
0.35 VLow, Logic 0
FIG 1 The low band, the gap and the high band of one example logic family. These band limits are an example from equipment data, not numbers to memorize.
Predict
Below 0.8 V is low. Above 2.0 V is high. You read 0.35 V, 4.6 V and 1.4 V on three inputs. What does the 1.4 V reading mean?
CautionA logic input measuring in the gap between the bands is a fault, not a valid state. A floating input (one connected to nothing), a broken wire or a partly shorted signal can park there, and the gate output becomes unpredictable. Never assume band limits from memory. They differ between logic families and between installations. Read them from the equipment maintenance data.
The AND Gate
An AND gate has two or more inputs and one output. Its output is high only when every input is high. If any input is low, the output is low. An AND gate works like switches in series with a lamp: the lamp lights only when every switch is closed.
AND: output 1 only when every input is 1.
FIG 2 Two switches in series with a lamp. Closed is 1, open is 0, a lit lamp is 1. Drawn from FAA-H-8083-30B, Figure 12-256.
A
B
Output
Tried
0
0
0
0
1
0
1
0
0
1
1
1
Try all four combinations of the two switches. 0 of 4 tried.
FIG 3 Simplified diagram of the AND logic. Source: FAA-H-8083-30B, Figure 12-256.
Source: FAA-H-8083-30B, page 12-121.
The OR Gate
An OR gate has two or more inputs and one output. Its output is high when any one input is high. The output is low only when every input is low. An OR gate works like switches in parallel with a lamp: closing any one switch lights the lamp. An annunciator, a labeled warning light on the panel, is often driven this way.
OR: output 1 when any input is 1.
Select a marked passage, or select Next, to read what it means.
The OR Gate
The example used is a “DOOR UNSAFE” annunciator. Let’s say in this case that the plane has one cabin door and a baggage door. In order for the annunciator light on the master warning panel to extinguish, both doors must be closed and locked. If any one of the doors is not secured properly, the baggage door OR the cabin door, then the “DOOR UNSAFE” annunciator illuminates. In this case, two switches are in parallel with each other. If either one of the two switches is closed, the light bulb lights up. The lamp is off only when both switches are open.
4 marked passages
Marked passages
Select a marked passage in the text, or select Next.
Source: FAA-H-8083-30B, page 12-122.
FIG 4 Simplified circuit that illustrates OR logic. Source: FAA-H-8083-30B, Figure 12-259.
FIG 5 OR gate. Source: FAA-H-8083-30B, Figure 12-258.
Check
Which circuit lights the DOOR UNSAFE light?
The NOT Gate
A NOT gate, also called an inverter, has one input and one output. Its output is always the opposite of its input: high in gives low out, and low in gives high out. Its symbol is a triangle with a small circle, called a bubble, on the output. The bubble is the important part: wherever a bubble sits on a gate symbol, it means inversion at that point.
FIG 6 Standard logic symbol and its possible logic states. Source: FAA-H-8083-30B, Figures 12-253 and 12-254.
FIG 7 An electronic circuit that reliably performs the NOT logic function. Each transistor is used as a switch, fully on or fully off. Source: FAA-H-8083-31B, Figure 11-62.
Read the Symbol
Select a card to turn it over.
Term
Active low
A signal whose active condition is the low level: the wire sits high and is pulled low when the condition occurs. A NOT gate turns it around for logic that needs a high.
CautionIf you miss an inverter or a bubble when you trace a diagram, every level after it reads backwards, and you will condemn a good card.
Sources: FAA-H-8083-30B, page 12-121; FAA-H-8083-31B, pages 11-25 to 11-27.
AND, OR and NOT Outputs
Two switches, A and B, feed an AND gate and an OR gate at the same time. A also feeds a NOT gate. Predict the three outputs for each setting, then check.
Setting 1 of 4
A = 1, B = 0
AND
OR
NOT A
Pick 0 or 1 for each gate, then select Check. The switches below move to the setting.
Free play
Set the Switches Yourself
Inputs
A0 B0
AND needs both. OR needs either. NOT A is the opposite of A.
AND
0
OR
0
NOT A
1
Still to see: AND = 1 with both on, OR = 1 with only one on, NOT A changing.
The Truth Table
A truth table lists the output for every possible combination of inputs, one combination per row. Nothing is left to memory. To build one, list every on and off combination of the inputs, with no combination twice, then work out the output for each row. Miss a row and you have missed a condition the airplane can be in.
Build the Table for the DOOR UNSAFE Light
A = cabin door not secured. B = baggage door not secured. Light = DOOR UNSAFE on. Each is 1 or 0.
Select a cell to change it between 0 and 1. Fill the A and B columns with all four combinations, then the Light column. Then select Check.
A
B
Light
Reading it backwardA truth table reads both ways. From the inputs you find the output. From an output you find every input combination that makes it: the DOOR UNSAFE light is on in three of the four rows, and off only when both doors are secured.
On many high-performance aircraft, a proximity sensor reports each gear’s position. A proximity sensor senses a metal part, called the target, near its face without touching it. It returns one voltage to the gear logic when its target is near and another when the target is far, and those two voltages are Logic 0 and Logic 1.
Select a number on the figure to read about that part.
Swipe the figure sideways to see all of it.
FIG 8 Proximity sensors are used instead of contact switches on many landing gear. Source: FAA-H-8083-31B, Figure 13-41.
Select a number
Each number marks one part of the figure.
Select a marked passage, or select Next, to read what it means.
Landing Gear Safety Devices
The use of proximity sensors for gear position safety switches is common in high-performance aircraft. An electromagnetic sensor returns a different voltage to a gear logic unit depending on the proximity of a conductive target to the switch. No physical contact is made. When the gear is in the designed position, the metallic target is close to the inductor in the sensor which reduces the return voltage. This type of sensing is especially useful in the landing gear environment where switches with moving parts can become contaminated with dirt and moisture from runways and taxi ways. The technician is required to ensure that sensor targets are installed the correct distance away from the sensor.
4 marked passages
Marked passages
Select a marked passage in the text, or select Next.
Source: FAA-H-8083-31B, page 13-22.
Check
The logic card reads 13 V from a downlock sensor. What does the card see?
Logic levels on an airplane are not one number. This sensor uses 0.3 V and 13 V, the autopilot inputs on the next page use 28 V, and circuit cards often use 5 V or less. The maintenance data gives the levels for the equipment in front of you.
Autopilot Engage Logic
An AND gate decides when a set of conditions are all met at the same time. The autopilot is a good example: it must not engage, or take control, unless every part it needs reports valid. Valid means the part has power and has checked itself as working.
Select a number on the figure to read about that part.
Swipe the figure sideways to see all of it.
FIG 9 AND logic of a system found in the aircraft wiring diagrams. Source: FAA-H-8083-30B, Figure 12-257.
Select a number
Each number marks one part of the figure.
Select a marked passage, or select Next, to read what it means.
The AND Gate
An example of AND logic could possibly be engage logic found in an autopilot. In this case, the autopilot would not be allowed to be engaged unless certain conditions are first met. Such conditions could be: Vertical gyro is valid AND directional gyro is valid AND all autopilot control knobs are in detents AND servo circuits are operational. Only when these conditions are met does the autopilot engage.
4 marked passages
Marked passages
Select a marked passage in the text, or select Next.
Source: FAA-H-8083-30B, page 12-121.
Check
The turn knob is not centered, so its input reads 0 V. Every other input reads +28 V. Can the autopilot engage?
Gates on the Aircraft
Each job below is done by one of the three gates. A master caution light, a single light that tells the pilot to look at the annunciator panel, is one of them: it lights when any one of the conditions it watches is present.
Read each card and put it with the gate that does the job. Drag the card, or tap a bucket.
AND
Every input high
Tap the bucket where the card belongs. On a computer you can also drag the card.
OR
Any input high
NOT
The opposite
Which bucket does this card belong in?
The Gear Warning Circuit
In many light airplanes the gear warning logic is built from switches, not from a circuit card. A limit switch is a spring-loaded switch that a gear part presses when the gear reaches the end of its travel. Each gear has an up-limit and a down-limit switch. A throttle switch closes when the throttle is pulled back below a set point. The horn sounds when the throttle is retarded and a gear is not down.
Select a marked passage, or select Next, to read what it means.
A throttle switch is also used in conjunction with landing gear circuits on most aircraft. If the throttle is retarded (closed) beyond a certain point, the aircraft descends and eventually lands. Therefore, many manufacturers activate a throttle switch whenever engine power is reduced. If engine power is reduced too low, a warning horn sounds telling the pilot to lower the landing gear. Of course, this horn need not sound if the gear is already DOWN or the pilot has selected the DOWN position on the gear switch. This same horn also sounds if the aircraft is on the ground, and the gear handle is moved to the UP position.
4 marked passages
Marked passages
Select a marked passage in the text, or select Next.
Source: FAA-H-8083-31B, page 9-53.
Select a number on the figure to read about that part.
Swipe the figure sideways to see all of it.
FIG 10 Aircraft landing gear schematic while the gear is in the DOWN and locked position. Source: FAA-H-8083-31B, Figure 9-95.
Select a number
Each number marks one part of the figure.
Sources: FAA-H-8083-31B, pages 9-51 to 9-53.
The Downlock Switch
The gear-down input to the warning logic often comes from a micro switch, a small switch that needs only a short push on its plunger to change over. It has three terminals: common (C), normally open (NO) and normally closed (NC). When the gear locks down, a part of the gear presses the switch’s lever, the plunger moves, and common connects to NO. Rigging is setting a part’s position so it works at the right point; a switch out of rig does not change over.
A micro switch. Source: FAA-H-8083-31B, Figure 9-168.
Contacts C to NCGear-down signal 0 V, low
Select a number
Each number marks one part. Then set the gear to Down and locked, and try Out of rig.
FIG 11 A micro switch. Source: FAA-H-8083-31B, Figure 9-168.
“A limit switch is simply a spring-loaded, momentary contact switch that is activated when a gear reaches its limit of travel.”
FAA-H-8083-31B, page 9-51.
Sources: FAA-H-8083-31B, pages 9-51 and 9-98, and the Glossary (microswitch). In this example common gets 28 V and NO feeds the warning unit.
Truth Table Rows
Each input can be 0 or 1, so each input you add doubles the number of combinations. The number of rows in a truth table is 2 multiplied by itself once for each input, written 2 to the power of the number of inputs.
Rows = 2 × 2 × 2 … one 2 for each input
FIG 12 AND gate logic symbols with two, three and four inputs. Source: FAA-H-8083-30B, Figure 12-255.
Worked example. How many rows does the truth table for the three-input AND gate need?
One input can be 0 or 1: 2 rows.
Add a second input. Each of the 2 rows splits into one with B = 0 and one with B = 1: 2 × 2 = 4 rows.
Add a third input. Each of the 4 rows splits again: 2 × 2 × 2 = 8 rows.
The three-input AND gate has 8 rows, and its output is 1 in only one of them: all three inputs 1.
Your turn. The autopilot engage AND gate in FIG 9 has 8 inputs. How many rows does its truth table have?
Hands-on
The Gear Warning Logic
On a logic diagram, the gear warning is a NOT gate feeding an AND gate. The gear-down signal is high when the gear is down and locked. The NOT gate turns it into gear-not-down. The AND gate sounds the horn only when gear-not-down and throttle-retarded are both high. Predict first, then set the switches to check.
Predict first
Gear retracted. Throttle at cruise power. Does the horn sound?
Gear
Throttle
FIG 13 The gear warning logic. A lit wire is Logic 1. The numbers are the level on each wire.
Gear down
Throttle retarded
NOT gear down
Horn
1
0
0
0
1
1
0
0
0
0
1
0
0
1
1
1
WarningNEVER disable or jumper a warning circuit to silence a nuisance warning. Find the fault. A defeated gear warning is a gear-up landing waiting to happen.
Applying the Procedure
Back to the horn that sounds with three green lights, after the warning unit was already replaced. Work it with these steps. Select a step to see what you find; at step 4, measure at each test point on the drawing.
What you find
Select a test point-.- V
FIG 14 The warning unit in the hook, with the gear down and locked and the throttle retarded. Each circle is a test point you can measure.
Decide
The gear-down input reads low with the gear locked, and the AND output is high. Which is it?
Check
Five questions, three answers each. Each answer tells you why it is right or wrong.
4 of 5
Needed to pass
1State the two logic states, the names used for each, and why a reading in the gap between the bands is a fault.
2Find the output of an AND, OR or NOT gate for any set of inputs.
3Build the truth table for a two-input gate and for a NOT gate feeding an AND gate.
4Decide from a gate’s inputs, whether a warning fault is a bad input or a bad gate.
Question 1 of 5
0 of 5
ResultsResults
Select any row to re-read its feedback.
Field Card
You can now:
1Read a logic level as Logic 1, Logic 0, or a gap reading that means a fault.
2Find the output of AND, OR and NOT gates for any inputs.
3Build a truth table that covers every combination the airplane can be in.
4Tell a bad input from a bad gate on a warning circuit.
States
Logic 1high, true
Logic 0low, false
Gapa fault, not a state
Levelsfrom the maintenance data
Gates
ANDevery input high
ORany input high
NOTthe opposite
Bubbleinversion at that point
Tables
Rows2 to the number of inputs
2 inputs4 rows
3 inputs8 rows
8 inputs256 rows
Troubleshooting
Inputs right, output wrongthe gate
An input wrongthe input
Thenfollow that wire
Neverjumper a warning
Read the gate, list what each input is doing, then decide: a good gate with a bad input, or a bad gate with good inputs.
Sources: FAA-H-8083-30B, pages 12-120 to 12-125; FAA-H-8083-31B, pages 9-51 to 9-53, 9-98, 11-25, 11-26, 13-22 and 13-23. The gear warning case is a training case.
