Tachometer

                            Tachometer

It works the same way a car tach works. It measures engine revolutions electrically from the coil/distributor or from a crankshaft sensor that is usually magnetic. On jet engines it is measured from the constant speed drive and is measured in percent of total power available rather than rpms.

A tachometer (revolution-counter, Tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine.^[1] The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The word comes from Greek Ταχος, tachos, "speed", and metron, "to measure".

  In automobiles, trucks, tractors and aircraft

 

Cessna 172's G1000 tachometer (1,060 RPM) and engine hours (1736.7 hours)

Tachometers or rev counters on cars, aircraft, and other vehicles show the rate of rotation of the engine's crankshaft, and typically have markings indicating a safe range of rotation speeds. This can assist the driver in selecting appropriate throttle and gear settings for the driving conditions. Prolonged use at high speeds may cause inadequate lubrication, overheating (exceeding capability of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring retracted valves) thus causing excessive wear or permanent damage or failure of engines. This is more applicable to manual transmissions than to automatics. On analogue tachometers, speeds above maximum safe operating speed are typically indicated by an area of the gauge marked in red, giving rise to the expression of "redlining" an engine — revving the engine up to the maximum safe limit. The red zone is superfluous on most modern cars, since their engines typically have a rev limiter which electronically limits engine speed to prevent damage. Diesel engines with traditional mechanical injector systems have an integral governor which prevents over-speeding the engine, so the tachometers in vehicles and machinery fitted with such engines sometimes lack a redline.

In vehicles such as tractors and trucks, the tachometer often has other markings, usually a green arc showing the speed range in which the engine produces maximum torque, which is of prime interest to operators of such vehicles. Tractors fitted with a power take off (PTO) system have tachometers showing the engine speed needed to rotate the PTO at the standardised speed required by most PTO-driven implements. In many countries, tractors are required to have a speedometer for use on a road. To save fitting a second dial, the vehicle's tachometer is often marked with a second scale in units of speed. This scale is only accurate in a certain gear, but since many tractors only have one gear that is practical for use on-road, this is sufficient. Tractors with multiple 'road gears' often have tachometers with more than one speed scale. Aircraft tachometers have a green arc showing the engine's designed cruising speed range.

In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT) side of the ignition coil,^[2] while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output. This is from a special connection called an "AC tap" which is a connection to one of the stator's coil output, before the rectifier. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) exist - usually on simple diesel-engined machinery with basic or no electrical systems. On recent EMS found on modern vehicles, the signal for the tachometer is usually generated from an ECU which derives the information from either the crankshaft or camshaft speed sensor.

 

                                                     CVR

Cockpit voice recorder

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Cockpit voice recorder (on display in the Deutsches Museum). This is a magnetic tape unit built to an old standard TSO C84 as shown on the nameplate. The text on the side in French "flight recorder do not open"

Both side views of a cockpit voice recorder

Cockpit voice recorder and flight data recorder; each with a ULB on the front.

Brazilian investigators found the cockpit voice recorder module of PR-GTD, a Gol Transportes Aéreos Boeing 737-8EH SFP, in the Amazon in Mato Grosso, Brazil. The airliner, while on Gol Transportes Aéreos Flight 1907, collided with a business jet, nosedived, broke up in-flight, and crashed.

A cockpit voice recorder (CVR), often referred to as a "black box",^[1] is a flight recorder used to record the audio environment in the flight deck of an aircraft for the purpose of investigation of accidents and incidents. This is typically achieved by recording the signals of the microphones and earphones of the pilots headsets and of an area microphone in the roof of the cockpit. The current applicable FAA TSO is C123b titled Cockpit Voice Recorder Equipment.^[2]

Where an aircraft is required to carry a CVR and utilises digital communications the CVR is required to record such communications with air traffic control unless this is recorded elsewhere. As of 2005 it is an FAA requirement that the recording duration is a minimum of thirty minutes,^[3] but the NTSB has long recommended that it should be at least two hours.^[4]

Overview

A standard CVR is capable of recording 4 channels of audio data for a period of 2 hours. The original requirement was for a CVR to record for 30 minutes, but this has been found to be insufficient in many cases, significant parts of the audio data needed for a subsequent investigation having occurred more than 30 minutes before the end of the recording.

The earliest CVRs used analog wire recording, later replaced by analog magnetic tape. Some of the tape units used two reels, with the tape automatically reversing at each end. The original was the ARL Flight Memory Unit produced in 1957 by Australian David Warren and an instrument maker named Tych Mirfield.

Other units used a single reel, with the tape spliced into a continuous loop, much as in an 8-track cartridge. The tape would circulate and old audio information would be overwritten every 30 minutes. Recovery of sound from magnetic tape often proves difficult if the recorder is recovered from water and its housing has been breached. Thus, the latest designs employ solid-state memory and use digital recording techniques, making them much more resistant to shock, vibration and moisture. With the reduced power requirements of solid-state recorders, it is now practical to incorporate a battery in the units, so that recording can continue until flight termination, even if the aircraft electrical system fails.

Like the flight data recorder (FDR), the CVR is typically mounted in the tail section (the empennage) of an airplane to maximize the likelihood of its survival in a crash.^[5]

Although it is commonly believed that Flight Data Recorders and Cockpit Voice Recorders are required on all US aircraft, in fact, they are only required on US aircraft that have 20 or more passenger seats or those that have six or more passenger seats, are turbo-charged, and require two pilots.^[6