Fuel Mixture Settings for Mountain Flying – Not a mystery

Posted by duewest
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Most pilots know the fact that atmosphere at high altitude is less dense than atmosphere at lower altitudes. They also know that warm atmosphere is less dense than cold atmosphere. Unfortunately, it seems to be a mystery to the pilots of many reciprocating, combustion engine propelled aircraft as to how to adjust the fuel mixture in their aircraft to operate efficiently at altitude. Pilot operating handbooks of these aircraft offer data or methods for setting the fuel mixture in the cruise segment of the flight. Many however don’t address fuel mixture settings for the phases of the flight that include engine starts, taking off from and landing at high altitude airports like Eagle (EGE), near Vail or Aspen (ASE).

Let’s cover a few basic ideas before we address the settings themselves. First of all, the reason for the atmosphere being less dense at higher altitudes is simple. Gravity. The atmosphere at sea level is supporting the weight of the atmosphere above. In other words, it is being compressed.

As you probably already know, in order to have a fire, three things must be present: Heat, fuel and oxygen. Notice the last item listed is “oxygen” and not “air.” Many of us forget that those two things are very different. On the planet which most of us live, the “air” around us is made up of roughly 21% oxygen and 79% other gases. The 79% of other gases is mostly of no interest when you are trying to build a fire. There is a mixture, or ratio, of oxygen and most aviation fuel that provides the most efficient combustion. (That means a good fire.) That ratio is 14.7:1 (oxygen:fuel). This is known as the stoichiometric ratio. You may still be able to have combustion at ratios that are a little different but not all of the particles will be completely consumed by the resulting combustion. For example, walk down the ramp at a mountain airport when several piston engines are operating and you will most likely find at least one that is snorting, popping and gurgling. It will also be exhausting puffs of black smoke. Those sounds and puffs of smoke are the engine trying to tell the pilot that the stoichiometric ratio is off. In other words, the mixture is off.

Notice I mentioned above that you “may still be able to have combustion at ratios that are a little different” than 14.7:1. This is true but the range seems to be more limited during engine starts and idling. If the aircraft is in motion and that movement is causing the prop to windmill, the engine valves are still be operated and the pistons are working like a pump to move fuel and oxygen in and out of the cylinders. In this scenario, the exhaust contains quite a bit of unburned fuel. This is exampled on a regular occasion when a piston aircraft lands at a mountain airport and just about the time the airplane slows to exit the runway, the engine stops. Not enough speed to windmill the prop and the mixture is too rich so the engine quits running! Unfortunately, most pilots are unaware of the problem and immediately try to do an engine start. Not realizing the engine is already flooded, (that means too much fuel and not enough oxygen, or too rich), the pilot starts cranking away while adding more fuel to an already too rich mixture. Next thing you know, they’re being towed from the runway and headed to the shop to figure out what’s wrong with the engine. (Truth be know, it was just a short between the headsets!)

If you find yourself in this situation, refer to the POH/AFM and use the manufacturers procedures for starting a hot and/or flooded engine. If you intend to fly into a mountain airport, you might want to familiarize yourself with this process before arrival. The runway might not be the best place to be reading.

Let’s think about what was going on before the aircraft mentioned above had the engine quit on the runway. The aircraft was approaching to land with too rich a mixture. In a non-turbocharged aircraft, a go around could have been a disaster. Especially when you consider that most pilots add full mixture right after full throttle. Again, making a bad situation worse. In a turbocharged engine, the turbo is compressing the air in the intake manifold back to sea level densities and so usually the engine will come to life. I say “usually” for two reasons: 1) the engine may require a little time to burn out the residual fuels and thus be somewhat under powered for a bit. 2) If the go around is attempted at a density altitude higher than the turbochargers ability to produce a sea level atmospheric pressure, the mixture may still be too rich. This second point is usually not going to be an issue because most turbochargers can produce sea level pressure up to at least a density altitude of 10,000 feet and usually more like 12,000 or 14,000 feet. With the exception of just a few mountain airports (like Leadville or Telluride), most of the airfield density altitudes will be within the turbochargers ability to produce sea level atmosphere in the intake manifolds.

While we are talking about turbochargers, we should mention that turbochargers have no, or little, beneficial effect until engine rpms are well above idle. In other words, for engine starts and idles (like landing roll) the engine is not being turbocharged and the mixtures need to be set as such. So, how can you know what mixture settings to use under these conditions? One suggestion would be, before heading to a mountain airport, over lower terrain, climb the aircraft to an altitude similar to the altitude of the airport of intended landing. Reduce power to a minimum safe setting and then adjust the mixture as you would in cruise flight using the EGT/ITT as applicable. Note this setting for use in landing at the mountain airport. Obviously temperature is an issue but you will generally find this obtained setting will be a workable beginning point.

Once at a mountain airport, starting can be a struggle for the uninformed. As mentioned earlier, and especially with Lycoming engines, flood the engine and use the manufacturers recommended procedures for starting under these conditions. At least you will be dealing with a known condition instead of guessing whether or not the engine is getting enough fuel. Whatever you choose to do, be careful to observe the starter operation time limitations. There should be no reason to burn up your starter or damage the engine flywheel when attempting high-density altitude engine starts. (Not to mention, you don’t want to start the flight with an exhausted battery. In most cases, you would be giving up your emergency electrical system backup.)

Here is a rule-of-thumb for you to try on your next visit to a mountain airport: You will know your mixture is correctly set for idle, if when you add power to obtain a 500 rpm increase, the engine sputters. In other words, you should have to add fuel to the mixture to get that higher rpm or else you were using too much fuel at the lower rpm. At idle, you should lean you mixture until you achieve the maximum rpm without adjusting the throttle.

The other issue we haven’t discussed is what mixture to use for takeoff. In a turbocharged aircraft, use full mixture. Remember the combustion part of the engine is being treated to sea level atmospheric conditions so you will use a sea level mixture setting. Unfortunately, the wings and propeller are still dealing with the thinner air of the higher density altitude.

For the non-turbocharged takeoff, use a mixture setting that gives you the maximum rpm when you have applied full throttle. Start with your mixture setting about 75% of full rich, advance the throttle to takeoff power, wait long enough to say “I love mountain flying” twice, and then slowly increase the mixture to see if there is an rpm increase. If no increase is noted, no further adjustment should be necessary. Pay attention to your EGT/ITT during the takeoff and climb to be certain they are within range. If too hot, add fuel to the mixture. With a little practice, it all becomes second nature.

If you still have questions about mixture settings and mountain flying, please drop us an email or give us a call. We would be happy to assist you.