Waypoints: Lean of peak savings quantified

April 1, 2009

Editor in Chief Tom Haines uses his Beechcraft A36 Bonanza for business and pleasure flights.

The controversy around lean-of-peak engine operations continues to fascinate me. The advocates of running an engine lean of peak exhaust gas temperatures are adamant that it not only reduces fuel flows, but is generally kinder to the engine because of lower peak pressure pulses in the engine and cooler cylinder head temperatures. Opponents speak even more stridently about the “dangers” of such operations, swearing that LOP fries exhaust valves and burns cylinders.

As with any other controversy, the truth lies somewhere in the middle. An educated pilot in a properly instrumented and equipped airplane can easily manage the engine for LOP operations and enjoy significant benefits. Probably somewhere, a pilot who didn’t understand how to operate an engine LOP and/or didn’t have the monitoring equipment to do so may have damaged an engine, and the naysayers apparently have latched onto that one case as gospel. As you’ll read in an upcoming issue, Senior Editor Dave Hirschman has not been able to find any data to support the notion that properly running LOP damages engines.

If you read this page regularly and my blog, “Reporting Points,” on AOPA Online, you know that I became a believer in LOP operations last year (see “Waypoints: Saving $4,000 in Fuel,” August 2008 AOPA Pilot). I have been flying an airplane equipped for such operations for a decade, but it wasn’t until fuel prices stretched to near $6 a gallon in 2008 that I began experimenting with LOP. Whatever hesitations I had over the years quickly gave way as I realized significant fuel savings and cooler engine temperatures with only a minimal reduction in cruise speed—usually about five knots.

In that August column, I noted that with fuel at $6 a gallon and flying my 1972 Beechcraft A36 Bonanza 150 hours a year, I could save about $4,000 a year in fuel. That assumes about 17.2 gallons per hour rich of peak and about 12.5 gph LOP. In a letter to the editor published in the October 2008 issue, a member noted that he was anxious to fly LOP, but his mechanic suggested that it would cause him engine problems and that by flying slower LOP any savings would be eroded to insignificance.

In an editor’s response, I said “rubbish” and explained that on a round trip between Maryland and Georgia flying LOP I saved about $208 in fuel while flying about 18 minutes longer than I would have ROP.

In an e-mail response to my comments, another member challenged my math. His point was that by slowing down, over time I would incur greater maintenance-related expenses on items that demanded attention based on hours flown, such as engine overhaul. “Perhaps flying more slowly to save on fuel costs might still be a good thing to do, but I suspect that the real, long-term numbers will not be as nice as what Mr. Haines presented,” the member wrote.

Doing the math

Hmmmmm. He had a point, but just how great might those “costs” be? I wasn’t sure, so I did some math (scary thing, I know) and found that in fact, the costs are almost insignificant compared to the savings. Considering fuel costs of about $4.35 a gallon, as they were early this year, and based on my 2008 usage, I would have saved about $1,627 in fuel while flying about 4.34 hours longer than I would have ROP. According to data from the aircraft price guide, Vref (which you can access from the Aircraft Ownership tab on AOPA Online), the “cost” of those extra 4.34 hours is $76.

Here’s where the numbers came from. My airplane was flown 124.1 hours last year on 30 trips. While I flight plan about 165 knots true airspeed, my average ground speed ROP of peak is probably about 145 knots. Let’s say that LOP the average ground speed is about five knots slower, 140 knots. That’s a 3.5 percent reduction in speed. All things being equal, that would mean about an additional 4.34 hours of flight time across those 30 trips, or a total flight time for the year of 128.4 hours.

In calculating the value of an airplane, Vref allows you to account for engine and airframe time. The default airframe value is an average number of hours based on the age and type of aircraft. Engine times are based on a mid-life engine. To adjust, you can enter your own actual airframe hours and actual engine hours. The calculated value will be adjusted up or down based on whether your airplane has more or less airframe time than average and whether your engine is more than halfway to TBO.

In the latest Vref update, each hour of airframe time for my airplane is worth $2.55. If it is below the average value, the value of the airplane will be increased by $2.55 for each hour up to the average; similarly, high-time airplanes will be devalued by $2.55 per hour above the average. Engine time for my airplane is worth $15 per hour. My engine is below the mid-point of the 1,700-hour recommended TBO and my airframe has fewer hours than average, so I get credit for that in valuation. That means that each hour flown costs me about $17.55 for engine and airframe time. So those extra 4.34 hours of flight time when running LOP versus ROP cost me $76.

Finding the savings

So going slower does have a cost, but what about the savings? If I average 17 gph ROP and fuel is $4.35, my hourly operating costs for fuel are $73.95. LOP at an average fuel burn of 13.5 gph (which is generous), the fuel costs drop to $58.73, a savings of $15.22 an hour. So a quick calculation shows that I could surpass the $76 in cost in only about five hours of flying. But I relish tedium, so let’s keep going.

To make those 30 trips last year, the airplane flew 124.1 hours. Let’s assume they were all ROP with an average fuel cost of $73.95. So my total fuel bill would have been $9,170 at an average fuel cost of $4.35 a gallon; the actual average cost per gallon last year was probably higher, but let’s keep things equal.

LOP the airplane would need to fly 128.4 hours to accomplish those 30 trips, which equates to a total fuel cost of $7,543, assuming $58.73 an hour as noted above—a savings of $1,627 over ROP operations. So it becomes an easy decision. I’ll spend $76 in flight time (and get to fly for an extra 4.4 hours to boot) to save $1,627 in fuel.

What’s not accounted for here is the extra efficiency of making fewer fuel stops. Several trips last year were nonstops because of the reduced fuel flow that comes with LOP operations. Particularly at 17-plus gph, the 74 gallons usable in my airplane makes for rather short legs. I could spend $10,000 adding tip tanks, or I can slow down a few knots LOP and stretch the endurance of the airplane by more than an hour. So on a couple of occasions, I saved the time associated with fuel stops, wear and tear on the gear system from the extra landing and takeoff, and wear and tear on the engine from an extra start cycle and especially the lengthy climb. We all know that the approach and takeoff phases of flight are statistically the most dangerous, so there is an increase in safety with flying fewer landings and takeoffs.

And don’t forget the environmental impact of burning less leaded fuel and reduced emissions when running LOP.

The bottom line is that if you have balanced fuel injectors, a multi-probe engine monitor, knowledge of your engine, and an engine that runs smoothly lean of peak, there is no reason not to run lean of peak. You will save significant amounts of money for a minimal investment in time and extra engine and airframe time. All of the additional time can be more than offset by flying longer legs with fewer takeoff and landings.

E-mail the author at thomas.haines@aopa.org.