Our recurrent flight training wouldn’t be complete without at least one seminar on personal minimums (limitations). It’s a serious concept for pilots to consider and realize personal flight minimums which might change based on one’s physical and mental condition. This is all well and good; however, it seems our training is often woefully deficient when it comes to understanding the limitations of the equipment we fly. To be more specific, we’re interested in those limitations listed in the Pilot’s Operating Handbook for engine operating parameters. These are FAA limitations have the potential to get one in trouble if ignored.
Some of these limitations include maximum Revolutions Per Minute (RPM), Manifold Pressure (MAP), Fuel Flow (FF), Cylinder Head Temperature (CHT), Oil Temperature (OT) and Oil Pressure (OP). I do realize that most pilots aren’t mechanics, data analysts or airplane geeks and are not going to immerse themselves into the inner workings of their airplane’s engine. Nevertheless, it is important for all to have at least a cursory understanding of these engine indications and what they can mean to the routine operation of the engine.
The number of times an engine makes a complete revolution is an essential part of every engine power setting calculation. The “red line” RPM is a bit unique in that it’s a limit as well as a target value. To get maximum power for take off, we must achieve that full red line value but not exceed it. To exceed that red line puts the engine and propeller in an unknown state for the pilot. Various design features of the engine establish a maximum rotational speed of the crankshaft. Not being an engineer I can’t list off or explain all of those design limitations, but I can imagine a couple.
As RPM increases, the effects of friction increase, but not at the same rate. Friction from torque increases with the square of RPM, but the power loss from friction increases with the cube of RPM. Some really smart engineer, or group of engineers, figured out at what point the maximum RPM meets a compromise with the friction losses to produce the maximum power.
The propeller is also designed for best high power performance when turning at the red line RPM. Anything less than that and all those takeoff charts in the POH become useless chatter. The proverbial fence and fifty foot tall tree won’t be passing under the plane.
Max manifold pressure in a non-turbocharged plane will be limited by the ambient conditions. All that power listed on the engine’s data plate presumes sea level standard day conditions. For those of us that don’t live near level, it’s always a strange little thrill when winter is upon us and we finally get to experience density altitudes at or below sea level just as a high pressure weather system moves through.
It’s important to remember, again, that just because you’ve pushed all the engine controls full forward, the engine probably isn’t making rated power. It is maximum power for the conditions, but that’s not the same as full rated power. Understand the limitation and plan accordingly.
Max manifold pressure in turbocharged aircraft is also a target red line for max take off power, similar to the RPM. Ambient conditions often make it a bit of a moving target. From one take off to another, you may see slight variations in the maximum pressure achieved, but it should be within an inch of the target.
Maximum fuel flow has a target red line on most engine installations as well. For legacy aircraft the limitation is actually a pressure limit that is calibrated to display as flow. Fuel flow is a combined function of RPM and MAP, so getting those two properly adjusted is a prerequisite to adjusting fuel flow.
Obtaining maximum fuel flow when at maximum power settings is a very important target. Our engines are cooled by air, oil and fuel. Having the sufficient fuel delivered is one of the most controllable by the pilot/owner and must be monitored at every flight.
When running at full power the engine is most susceptible to detonation. Getting plenty of fuel into the system is the primary mitigation against destructive detonation. One indicator of this danger is high CHTs. Watching for maximum FF at full power situations is a significant safety item.
Max CHT is just the opposite of a target number and should be avoided at every opportunity. It’s a matter of cylinder longevity to hold CHT below 400. Allowing it to reach the 460 degree F red line should tell anyone that something is terribly amiss.
Most of those reading this magazine are very familiar with the concept of the aluminum cylinder head’s structural integrity when temps exceed 400 degrees F. It’s also important to know that any sudden departure upward from normal temps is something to investigate.
Don’t misunderstand, having very cold CHT isn’t a target either. A natural part of developing power in a reciprocating internal combustion engine is heat. Temps down in the mid 200 range may likely be indicating something is amiss in the combustion chamber.
Turbine Inlet Temperature is another top end temperature limit to avoid. Even though many POH tolerate exceedences for short periods, some accelerated wear still occurs. There are probably many reasons not to exceed TIT limits, but two come to my mind.
The metals used in the exhaust and turbocharging system have structural temperature limits. Going past these limits puts the operator into a test pilot and experimental mode that is best avoided. Continual and/or long term excursions past red line will weaken the metal and allow premature erosion which leads to cracks.
Cracks lead to loss of pressure in the exhaust system which reduces energy to turn the turbocharger which reduces available manifold pressure which lowers the engine’s critical altitude abilities. If the plane is typically flown at lower altitudes, this loss of capability may not be noticed. The danger exists nonetheless.
When a crack occurs in an exhaust system, it can allow a hot “jet” of exhaust exiting like a torch. The worst of it isn’t the loss of performance but not knowing toward what component in the engine that torch is aimed. Keeping TIT below red line won’t eliminate exhaust failures, but it helps.
OT & OP
Oil temp and pressure are interesting in that their limits are often intertwined. Higher oil temps usually return lower oil pressures, useful knowledge for in-flight troubleshooting. Also, very low oil temps can easily drive oil pressures up to red line. Winter operations are particularly difficult when oil temps need to get above 100 degree F before maximum RPM is applied. Otherwise, excessively high oil pressures might damage seals or stress the oil pump.
Continental engines like to run oil pressure right around 50 p.s.i. when oil temps are 180 degrees F and RPM at cruise settings. Lycoming engines do better at 80 p.s.i. In the same situation.
On those hot days don’t be surprised or overly concerned if you see very low oil pressures after landing and taxiing at low RPM. With oil temperatures at 180+ and less than 1000 RPM, the very thin oil isn’t capable of producing much pressure. Continentals might show as low as 10 p.s.i. and Lycomings down around 30 p.s.i. in this situation, and it’s perfectly normal.
If asked, many pilots can’t state the allowable or typical parameters for many of the engine indications. To add to that, they may also not understand what affects those indications or what actions might be needed to control them.
To find a pilot’s level of understanding, I may ask if the engine is getting full power indications for take off. The answer is almost alway yes, but the sometimes blank look in their eyes tells me they may not fully appreciate the question. A quick look at the engine monitor data usually confirms my expectations. A follow up question is whether the RPM, MAP and FF are making it to full red line during take off. Many can’t answer this question either, except to reply that “all the levers are full forward and the engine feels strong”.
To fly safely and successfully it’s important to know the engine/aircraft is performing as it should for the situation. If one expects to achieve take off flight performance by the book, then one must also know if the engine is doing its part in the effort. The same goes for climb, cruise and descent. The first effort is on you, the pilot, to get out those books again, find what the limitations are, and put them to good use.