Wednesday, June 29, 2005

Engine Instruments

When you look in the cockpit of an airplane and are taken aback by the bewildering array of instruments, part of the problem is that a good proportion of the instrumentation is doubled or more. The engine instruments are multiplied by the number of engines. The flight instruments are multiplied by two for two pilots, sometimes with tertiary redundancy in standby instruments. There may also be more than one of the same navigational instruments. Faced with this array, the casual observer can be left unable to identify something basic, like a clock.

The engine instruments are usually right in the middle of the panel, in a column n wide, where n is the number of engines. I'm going to go over what I need to know about the engine instruments on my beastie.


The torquemeter indicates the pressure being delivered to the propeller reduction gear. It's marked in psi with a red line at the maximum of 42.5 on the earlier models and 50.0 on the later ones. It is driven by a cylinder assembly mounted on the rear reduction gear. Gearcase oil pressure is delivered to the cylinder. Increased torque drives a piston which increases the oil pressure in the cylinder. The oil pressure is compared to gearcase pressure, in order to compensate for a decrease in pressure with altitude, and the difference transmitted (1A 26V AC) to the cockpit gauge. If AC power fails, the indication drops to zero.

These are the top centre gauges on the dashboard, right under the fire handles. Engine torque is the main measure of engine power, but to actually determine horsepower output, propeller speed must be considered.

Propeller RPM

Propeller RPM or Np is reported by a tachometer located at the front of the reduction gear. It generates its own power, so no aircraft electrical buses are involved. It is indicated on gauges right below the torquemeters, measured in percent, even though 100% doesn't necessarily mean anything, as max rpm is 100% on the early models by only 96% on the newer ones. The gauge shows marks every two percent, with the numbers displayed and larger marks every ten percent. There is also an inset gauge that shows 0 to 9 around in one circle, indicating 1% intervals more clearly than the little hash marks. It is important to set rpm precisely.

Shaft horsepower is obtained using the formula:

SHP = (Np x torque) / 172.77  (for the PT6A-27)


SHP = (Np x torque) / 170   (for the PT6A-20)

The numbers 172.77 and 170 are the thermodynamic constants for the respective engine models, along with whatever is needed to make the dimensions of SHP work out. And you know that when they give the divisor to five significant figures that you'd better not set the torque to only three sig figs. "38.671 pounds torque set, captain, sir!" Um, yeah. There's also a custom made circular slide rule that will help do the calculations, but I don't have one. Yet.

If you're beginning to get the idea that you don't conduct a takeoff in this airplane by shoving the power levers up to the firewall, or even up to the red lines, you're right. The PT6A-27 is derated for this airplane, meaning that you act as though it's a 620 hp engine when it's actually capable of 680 hp. (If you used full power, and one of the engines failed, the airplane would be uncontrollable.) You calculate the appropriate setting and set that at takeoff, whether or not the power levers can still go further forward. If you cannot achieve the torque required without exceeding T5 or NG limits, then you have a maintenance problem and you go take a coffee break while someone in coveralls deals with it.


This is the temperature at station 5 of the engine, between the power turbine and the compresser turbine. As I mentioned before, it's not the hottest zone of the engine, but it's much easier to measure temperature here. Sort of like the old joke about the drunk looking for his keys near the lamppost. Under test conditions it is possible to measure the temperature in the hottest part of the engine, station 4, where the maximum is 1050 degrees. A relationship was found between there and T5, so that by observing T5 you can accurately predict the corresponding T4. So the pilot memorizes this:

Phase of FlightPT6A-20 T5 maxPT6A-27 T5 max
Takeoff & Single Engine Emergency750725
Max Climb725695
Max Cruise705695
Max Reverse750725

T5 is measured via eight thermocouples wired in parallel, and is powered by the voltage produced by the temperature difference across the probes, so requires no aircraft electrical power.


This is the rotational speed of the gas generator, the rear shaft of the engine. It is sensed by a tachometer generator driven by the accessory gearbox, at the rear of the engine. Typing "accessory" just gave me a flasback to grade eight English, when the teacher insisted that accessories meant items that complement a woman's outfit, not "extra stuff you get, like stuff to make your car better." I don't suppose he would have been any more receptive to "extra equipment that runs off an engine drive shaft, like pumps and tachometers." The scale works the same way as the RPM gauge. You just have to remember that this one is at the bottom, under the ITT gauge.

Fuel Flow Gauges

These run off 26V AC and show fuel flow in pounds per hour. The fuel flow transmitters are on the fuel lines above the fuel strainers and before the fuel emergency shutoff valves. If AC power fails, the indication freezes.

Oil Temperature

Left and right DC buses power oil temperature gauges, right above the oil pressure gauges. The probes are on the accessory gearcase.

Oil Pressure

Each oil pressure gauge is two gauges in one. The first reports pressure measured at the accessory gearbox and transmitted by 26V AC to a needle that travels over a circular gauge. AC failure results in a zero pressure indication. Coloured bands show pressures acceptable at idle power only (yellow, 40-80 psi) and at engine speeds 75% NG and above (green, 80-99 psi). A pressure sensor located near the oil cooler will illuminate a 28V DC low oil pressure caution light if the pressure drops to 40-42 psi. The light goes out again if the pressure rises to 44-46 psi.


aasmodeus said...

any info as to how the fuel flow transmitters work? if the ac is cut off, do they in any way impede the flow itself (physical reading, e.g. propeller)?

Anoynmous said...

Ooh, an inconsistency! I thought AC-powered instruments hold their indication if power is lost, but you say the oil pressure gauge drops to zero like a DC-powered instrument would. I guess in this case AC lies...down and plays dead.

Aviatrix said...

aasmodeus: I don't know exactly how the fuel flow transmitter works. From this description of the overhaul process of a fuel flow transmitter and this catalogue of flow measurement methods, I'm going to guess that there is an impeller or other mechanical device inside which has a magnetic connection to a pickup on the outside. The gauge freezing doesn't correspond to the shaft of an impeller freezing. Fuel flow continues unabated and I know that one from experience.

anoynmous: yeah, I know. Perhaps there is special engineering in those gauges, because trust in lies about oil pressure or torque would be especially misplaced. Or perhaps the mnemonic is just poor.

aasmodeus said...

Cool beans, thanks. Although you need to edit the "this description" URI; it should stop at "FF_OHC.pdf".

Aviatrix said...

Aargh. Can't edit comments. It'll have to stand. ANyone smart enough to understand the link has to be smart enough to edit a bogus URL.

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