Land and Hold Short

Sacramento Sky Ranch, which sells airplane parts, has an RSS 2.0 feed dedicated to light aircraft maintenance, including pictures and even sound.

There’s lots of great stuff here, from the sound a cracked cylinder makes when you flick the fin (yes, there’s a WAV file!) to why an oil/air separator to an engine is (in their opinion) an incredibly stupid idea. It’s aimed mainly at mechanics, but there’s good information for the owner/pilot as well. I’ve subscribed.

Student pilots and renters rarely worry much about the red knob or lever that controls the fuel/air mixture to their engines; owners worry about it a lot. For a while, there has been a big controversy about how far to lean engines: as the fuel/air mixture gets leaner (more air/less fuel), each cylinder’s exhaust gas temperature (EGT) gets higher and higher; then, if you keep leaning, the EGT drops again. The hottest possible exhaust temperature is called peak EGT. If you lean the mixture at peak EGT (i.e. reduce the fuel flow), the EGT drops and you are running lean of peak, or just LOP; if you enrich the mixture at peak EGT (i.e. increase the fuel flow), the EGT also drops and you are running rich of peak, or ROP. Which is better?

The LOP vs. ROP debate was pretty ferocious for a few years, but the LOP pilots seem to be gaining ground — after all, isn’t it better to cool your engine by burning more (free) air and less fuel, than by burning more fuel and less air? Besides, extra, unburned air doesn’t gunk up the engine, while extra, unburned fuel leaves all kinds of nasty stuff behind, as well as producing a significant amount of carbon monoxide (a big threat to winter fliers like me). Aside from the cost of fuel and the risk of CO poisoning, however, there’s also the question of engine health; after all, an early overhaul will cost a lot more than a bit of extra fuel.

Running Hot and Cold

The previous paragraph already mentioned gunk from flying ROP — for example, lead deposits that foul spark plugs — but the biggest threat to an engine is heat, not in the exhaust gas but in the cylinder itself. Many planes, including my Warrior, do not have a gauge installed for measuring cylinder head temperature (CHT), and most of those that do have a probe in only one of the four or six cylinders. The opponents of LOP (including some less-than-educated mechanics) used to claim that flying LOP increased cylinder temperatures and thus shortened cylinder life. In fact, it turns out that peak CHT — the hottest possible temperature inside the cylinder — actually occurs during ROP flight, specifically when the exhaust temperature has fallen about 25-50 degF on the rich side of peak EGT. In other words, if you lean your engine to peak EGT and then enrich slightly, you will be closer to peak CHT (and to damaging your engine); if you lean your engine to peak EGT and then lean a little further, you will be further from peak CHT. The chart that demonstrates this should be present in any engine operator’s manual (I’m using the one on page 3-13 of the Lycoming Operator’s manual for the O-320 and IO-320 series), but John Deakin also has one online here that shows the same thing (after you stare at it for 45 minutes or so; the best source of information on LOP operations, by the way, is John Deakin’s columns on AvWeb — see engine-related columns in the sidebar).

Bad Vibrations

So why not always fly LOP? One problem is that some engines just cannot do it. The fuel/air distribution to the cylinders is not always even, so one cylinder might be running much richer than another; by the time you lean far enough to get the richest cylinder LOP, the leanest cylinder might no longer have enough fuel to ignite at all, and the engine will start vibrating violently. Carbureted engines mix the fuel and air together in a single place (the carburetor) then send the mixture to all of the cylinders, where it arrives in various states (sometimes more air will get through, and sometimes more fuel). Fuel-injected engines actually mix the fuel and air separately for each cylinder, so it should be possible to adjust them so that all cylinders get exactly the same mixture.

In fact, the original factory injectors almost never work that well, but GAMI makes third-party injectors that do a much better job; not surprisingly, the company and its founder, George Braly, are strong advocates of flying LOP. Lycoming, one of the two major engine manufacturers, has just as strongly opposed LOP, using articles like this one. Essentially, Lycoming’s argument is that with a constant-speed propeller pilots have no way to read their power setting directly, so if they lean the mixture and increase manifold pressure to compensate for the lost power, they might end up flying lean at a dangerously high power setting. That argument does not apply to engines with fixed-pitch propellers, like the one on my Warrior, because there the power setting corresponds directly to the RPM at any given density altitude (or, in plain English, it’s no harder to figure out the power setting LOP than ROP). Even with a constant-speed propeller, the same horsepower should produce the same indicated airspeed no matter where the mixture is set, so it’s not that hard to manage the power setting.

Wide-Open Throttle!

The most fanatical faction of the LOP group — and the one to which I belong — is the group that flies lean of peak/wide-open throttle (LOP/WOT). Using this technique, you do not touch the throttle at all until you’re descending for landing; instead, you leave the throttle wide open the way it was for takeoff, and then you use the mixture (red button or lever) exclusively to control power, going leaner to reduce power, or richer to increase it. That way, you’re always flying the leanest possible for any given power setting (you cannot open the throttle any further to get more air), so there’s no hard brain-work involved. Of course, you need an engine that runs well LOP to pull this off, either a fuel-injected engine with GAMIjectors or a four-cylinder carbureted engine with good distribution like the O-320 (a six-cylinder carbureted engine is unlikely to work, because it’s impossible for all six cylinders to be the same distance from the carburetor). The fuel savings can be spectacular: I burn about 20% less fuel in my Warrior flying just as fast, and I have cleaner plugs and minimal risk of CO poisoning if the muffler ever leaks into my cabin heater. In fact, I am especially fortunate, because in the early 1980’s, Piper’s Warrior II POH actually recommended LOP/WOT far ahead of its time:

For Best Economy cruise, a simplified leaning procedure which consistently allows accurate achievement of best engine efficiency has been developed. Best Economy Cruise performance is obtained with the throttle fully open. To obtain a desired cruise power setting, set the throttle and mixture control full forward, taking care not to exceed the engine speed limitation, then begin leaning the mixture. The RPM will increase slightly but will then begin to decrease. Continue leaning until the desired cruise engine RPM is reached. This will provide best fuel economy and maximum miles per gallon for a given power setting. See following CAUTION when using this procedure.

CAUTION

Prolonged engine operation at powers above 75% with a leaned mixture can result in engine damage. While establishing Best Economy Cruise Mixture, below 6,000 feet, care must be taken not to remain in the range above 75% power more than 15 seconds while leaning. Above 6,000 feet the engine is incapable of generating more than 75%.

For my Warrior’s 160 hp O-320-D3G Lycoming engine, it seems to be RPM rather than power setting that determines things: my engine will almost always run smoothly LOP/WOT at 2500 RPM or above, and will sometimes let me get down to 2400 RPM. Obviously, then, I do better at higher density altitudes, where these RPMs give me safe power settings.

Last Word to Lycoming

Despite all of the company’s opposition to GAMIjectors and LOP, at least someone at Lycoming seems sympathetic. The January 2005 issue of COPA Flight reprinted an article “Carburetor or fuel injection system?” from a past issue of the Lycoming Flyer newsletter (no date provided). Essentially, the article says that the advantages of a carburetor are cheap installation and maintainance as well as wider fuel lines that as less likely to plug up; the advantages of fuel injectors are no carb ice, the ability to fly inverted (for aerobatics), and improved fuel/air distribution, which

allows leaning that results in slightly lower overall fuel consumption. This is of particular value in higher horsepower engines where saving a small percentage of the fuel being burned may result in a significant dollar savings.

But wait … carbureted engines already have good enough distribution to fly a bit rich of peak. Is Lycoming suggesting that it’s possible lean further than that, perhaps to peak, or even lean of peak? I’m guessing that someone’s been brainwashed by GAMI’s marketing material. In the meantime, I’ll keep on flying little four-cylinder carbureted O-320 LOP/WOT until I find a good reason not to.

A comment by a fellow pilot got me thinking about headwinds and tailwinds. I started flying with serious misconceptions about how a headwind or tailwind affects a flight, and some of the bogus rules of thumb only makes things worse. I’m going to take a quick look at three popular misconceptions here — that you make up for a headwind on the return trip, that your average groundspeed over many trips is a good indication of your plane’s true airspeed, and that you should fly faster into a headwind to save fuel.

Making up for a headwind

First, the easiest one. Let’s say that you’re flying on a round trip in a single day, and the wind is forecast to be the same all day. On your way outbound, you will have a headwind, and on your way back home, you will have a tailwind. Sounds about even, right? To try it out, consider a plane with a 120 kt true airspeed (like a Warrior or a Cessna 172) flying 300 nm each way:

In other words, you never make the time up, because you spend more of the flight in the headwind than the tailwind, by definition. There are tricks, of course, like flying low westbound to get a weaker headwind and flying high eastbound to get a stronger tailwind, but averaged over many flights, the best wind is still no wind at all.

Average ground speed

And that comes to the second point. When people want to challenge the true airspeed figures put out by the airplane manufacturers, they often pull out their average groundspeeds, which are inevitably 10 knots slower or more, for which they usually blame the manufacturer’s marketing department. Part of the difference can be explained away by density altitude (nobody always flies at 7,000-8,000 feet density altitude), low power settings (many pilots are shy about setting 75% power), or climb, approach, etc., when the plane is flying outside its optimum conditions, but another big part of the difference is the wind. For example, in the table above, the plane’s average groundspeed would be 120 kt with no wind, 117 kt with a 20 kt wind, or 107 kt with a 40 kt wind. Average groundspeed is a more accurate indication of how long trips will take in a plane, but it is a different measurement than the plane’s true airspeed, because any wind at all hurts the average, and there’s almost always some wind.

Fly faster into a headwind

Flying faster into a headwind will definitely get you home sooner, but it won’t usually save gas, despite what many pilots, including flight instructors and textbook writers, try to tell you. To demonstrate, I’ll use another table. At 8,000 ft density altitude, a Cessna 172p will fly 121 ktas burning 8.6 gph at 75% power, 112 ktas burning 7.4 gph at 65% power, and 100 ktas burning 6.2 gph at 55% power according to its POH; this table shows the time and fuel to fly 300 nm with different headwinds:

Obviously, if you just want to get home, you are better off speeding up and paying for the small amount of extra gas–you’d have to be pretty dedicated to fuel savings to make your trip an hour longer to save 1.5 gallons of fuel. However, if you’re worried about running dry and there’s nowhere to turn around to (let’s say that you’re halfway between Greenland and Iceland), speeding up is not necessarily the best choice. Even at 55% power, you’re still burning half a gallon less gas than at 65% power and almost two gallons less than at 75% power flying directly into a 30 kt headwind. With a 40 kt headwind, speeding up to 65% power starts to make sense, but 75% will still burn a full gallon more fuel; in fact, you need to get up to a 60 kt direct headwind before you will save fuel by speeding up to 75% power.

Note that these numbers are for a very slow plane. If you’re flying a fast single, like a Lancair or Cirrus, or a twin, you will probably always be considerably better off at 55% for conserving fuel, unless you’re flying into a hurricane. I imagine that cross-ocean ferry pilots pretty-much always fly at low power settings, no matter what the wind is like.

Last week, I flew my first flight for Hope Air, a charity similar to Angel Flight in the United States and British Columbia. An icy, snow December is a strange time to start on something like that — many of the singles have been put to bed for the winter — but it was a nice, easy introductory baby trip from Ottawa to Toronto, and I had the benefit of knowing that Frank Eigler was waiting by his cell phone to charge to my rescue in his ice-certified Aztec if my little Warrior got stuck anywhere.

I’ve always wanted to help people with my time as well as money, but I had enough of stuffing envelopes during my teen years. Hope Air looked like a great opportunity, but only this year did I finally have enough hours to meet their requirements. They have a tough screening process, and I was happy to make it through and get my little baseball cap in the mail. The trip went well — I won’t print personal details about the patient or her escort, except that they were wonderful, friendly people — and I felt more like I was taking friends or neighbours for a ride than performing any act of charity. Frank was at the Toronto Island airport to meet me, and he gave me a city tour in his Aztec followed by some engine-out practice over Lake Ontario to celebrate my first Hope Air flight. I flew back late in the afternoon, and landed uneventfully despite a burned-out landing light (legal, as long as there are no passengers). My next flight will be to Kapuskasing in January, a much longer trip where the weather will have to be just right.

If you live in Canada, have the experience required, and are retired, self-employed (like me), or have an employer who is flexible about hours (like Frank, who works for RedHat), I highly recommend the Hope Air organization.

My Warrior is one of the slower planes on the apron. It’s not as slow as some people claim, of course — under ideal conditions, I actually can get within 2-3 knots of the 127 knots true airspeed promised by the POH — but it makes long work of short trips compared to (say) the Mooneys or Barons, not to mention the new Cirrus and Lancair planes. I thought it would be interesting to find out just how big that difference is in real life, and what the cost is, so I plugged the best performance numbers I could find for a bunch of light aircraft into a spreadsheet, and figured out cruise time and fuel for a 400 nautical mile trip with no wind, a 20 knot headwind (normal for a low-altitude westbound trip), and a 20 knot tailwind (normal for a low-altitude eastbound trip). The results follow.

No Wind

As far as I can determine, these are all performance numbers for the plane’s optimal altitudes (depending on the engines). Unfortunately, I have not been able to find good numbers for the Lancair Columbia, so I’ve left it out: its performance should be similar to but slightly better than the SR-22. For a 400 nm trip, ignoring taxi, climb, and descent, here are the numbers:

Some of the slower planes are surprisingly fuel efficient in this table: for example, the Cessna 172 and the Piper Warrior are almost as fuel-efficient as the Mooney 201, though they take a fair bit longer to complete the trip. The range of fuel efficiency is quite large: from 6.8 nm/gal for the Seneca, to 16.9 nm/gal for the TwinStar.

20 kt Headwind

A headwind should improve the relative fuel efficiency of the faster planes, since they spend less time in it than the slower ones. It will also greatly increase the time spread between the fastest and slowest planes:

At this point, the slower planes (including the Cessna 182) really start to suffer. With 40 gallon tanks, the Cessna 172M is pretty-much at the limits of its fuel reserves for this trip; the Warrior, with its 48 gallon tanks is still safe (probably even for IFR), but both make for a very long trip. The Seneca is now almost twice as fast as the Cessna 172 and Warrior. Note, though, that the Baron still has only a half hour advantage over the Mooney, while burning more than double the fuel.

20 knot Tailwind

A tailwind should eliminate some of the advantage of the faster planes: they will burn more fuel, but won’t get you there all that sooner:

Reflections

All of these numbers are a little misleading, of course. For example, the first part of any trip is spent climbing, and a plane that climbs slowly (like the 172M or the Warrior) will spend relatively longer than a plane that climbs fast (like the twins), slowing it down a bit. There’s also the matter of maneuvering around weather enroute, long vectors for approaches at the destination, and so on. All of these planes, then, will take a little longer for the trip than these numbers suggest — my Warrior, for example, more typically needs 4:00 takeoff to landing for a 400 nm westbound trip with a moderate headwind, and 3:00 for a 400 nm eastbound trip when you factor all of that in.

The twins are not much faster than the high-performance singles but burn a lot more gas. Of course, they have other benefits, such as a redundant engine and (often) deicing equipment, but those come at a very high price. The one exception is the TwinStar, which actually outperforms its single-engine sibling both on speed and fuel burn.

So, in practical terms, what would it mean to me to upgrade to a faster plane, like a Mooney 201 or even a Cirrus? On this hypothetical 400 nm round trip with a headwind outbound and a tailwind inbound, my Warrior uses 6:38 flying time and burns 55 gallons of AvGas (about USD 190.00 at current fuel prices). A Mooney 201 would use 5:03 flying time and burn 53 gallons: that’s a hour and a half faster and about USD 10.00 less fuel to boot. A Cirrus SR-22 would get me there and back in only 4:30, for a two-hour saving, but would burn 74 gallons, adding about USD 90.00 to the fuel cost. In other words, the Cirrus saves only a half hour over the Mooney at a cost of USD 100 extra in fuel. Most of the twins are too expensive to even bother calculating, except for the TwinStar, which manages the trip in the same time as the Cirrus using even less fuel than the Mooney.