Should You Trust Your Engine Instruments?

Paul McBride, a former Lycoming employee widely known as “Mr. Lycoming,” likes to tell the story of the Mooney owner whose preowned Lycoming IO-360-powered single wouldn’t get up to the specified maximum RPM setting of 2700. After an adjustment at a shop, the engine did obtain 2700 RPM on the gauge, but the power plant failed in flight shortly thereafter.

After a safe emergency landing, Canadian authorities took over and found that a connecting rod had failed. Contributing to the failure was a manifold pressure gauge that was reading two inches low – not an unusual error, according to McBride – but more importantly, the tachometer that was reading 500 RPM too low. The error meant that the engine had been turning at 3200 RPM, a factor that ultimately led to the failure.

The point of McBride’s story is not to try to assign blame, but to accent the dangers of having blind faith in the accuracy of engine instruments. Many owners have such faith: McBride recalls seeing engines that had been overhauled three or more times, with owners reinstalling the original engine instruments each time. Lycoming strongly recommends getting engine instruments calibrated annually (see Service Instruction No. 1094D, Fuel Mixture Leaning Procedures, and SSP 400, Operating Recommendations for the TIO-540-AE2A engine). It’s important to note that engine gauges are not considered part of the engine proper. In fact, FAA-approved data covering the verification and calibration of the instrumentation is the responsibility of the airframe manufacturer, not the engine maker.

In between overhauls, it’s up to the pilot in command or the maintenance shop to spot calibration errors. Aviation maintenance technicians, as part of a 100-hr. or annual inspection, should scan the gauges for proper operation during a pre-inspection run-up. If there’s a problem, gauges and/or sensor units would likely be swapped out or sent out to an avionics shop or repair facility for recalibration or fixes. Once the system is returned, Lycoming recommends that the gauge, sensor, and interconnect wiring be calibrated by a qualified technician or agency before flight.

"Of the typical engine instruments (tachometers, oil temperature gauges, oil pressure gauges, cylinder head temperature gauges, exhaust gas temperature, manifold pressure gauges and turbine inlet temperature probes for turbocharged engines), tachometers are the most notorious for being out of calibration," says McBride. "Calibration errors as small as 5% or 10% in RPM reading will greatly increase the load on the propeller and the engine bearings during operation,” he says. Even off-the-shelf instruments are not beyond reproach. McBride says one maintenance facility owner told him that he’d once tested five tachometers off the shelf and found that each was indicating about 150 RPM low. That’s an error of more than 5% of the cruise RPM of many engines.

An instrument expert with Keystone Instruments in Lock Haven, Pennsylvania, says an installation error as innocuous as cutting a tachometer cable ¼-inch too long can cause the instrument to read 500 to 600 RPM fast; worse yet, using the wrong cable can damage the tach. Usually though, he says problems arise when the inside of the gauge gets dirty and needs to be cleaned. Either way, operators are required to send gauges to an authorized maintenance facility for repairs or recalibration, according to Keystone.

Figuring out whether gauges are calibrated correctly in the field is sometimes obvious, sometimes not—a la the Mooney mishap. An oil temperature gauge, for instance, should read the ambient temperature before the engine is started. Manifold pressure, too, should match the ambient pressure before start.

"Calibration errors as small as 5% or 10% in RPM reading will greatly increase the load on the propeller and the engine bearings during operation,” he says.

Some gauges can be operationally verified on the cheap by a local maintenance shop. Ben Visser, an aviation columnist and former Shell Oil chemist, recommends checking oil temperature gauges by placing the sending unit in a pan of oil sitting on a hot plate. Using a thermometer, Visser says to heat the oil to 180˚ F then to mark the gauge with a permanent marker, regardless of whether the gauge has temperatures printed on the face.

Why 180˚? Visser explains that the peak oil temperature in a normally aspirated engine is typically 50˚ F higher than the temperature of the oil in the sump, the location of the sensor. The peak temperature in a turbocharged engine is about 70 to 75˚ F hotter than the indicated temperature. He says the typical “green band” on an oil temperature gauge ranges from 120˚ F to 245˚ F.

By calibrating the gauge and operating near the 180˚ point in cruise, Visser says the oil temperature at the hottest point in the engine will exceed 212˚ F, boiling off any water that has accumulated in the oil. By consistently operating below the boiling point, Visser says water and acid can build up in the crankcase, leading to rust and corrosion and reduced engine life. Keystone sometimes checks its oil temperature gauges by immersing the sending units in boiling water to verify the gauge reads 212˚ F.

The arrival of solid-state integrated avionics and liquid-crystal displays for the cockpit, while eliminating the ability to strike a line on a gauge face, will undoubtedly boost reliability and readability of engine gauges. Popular systems like Blue Mountain Avionics’ EFIS/One “glass” cockpit for experimental aircraft allows an operator to display as many as 16 different engine gauges on-screen, whether the input from each is voltage, differential voltage, resistance or thermocouple. Calibrating the unit is relatively simple, too. The unit has a special screen that allows the operator to view and modify the setup and calibration information for each sensor.