Headwinds, tailwinds on cross-country flights

First, many thanks to COPA reader David Megginson on his Pilots to Pilots comments in the November 2005 COPA Flight regarding time and fuel savings on longer flights.

My comments in the October issue with respect to the use of lower descent rates for efficiency related to VFR flight since most of our member’s aircraft operate VMC.  Nonetheless, unless one is in controller-manipulated airspace, it is often possible to make planned descents in the 200-300 fpm rate to initial approach fixes from higher altitudes –especially when one asks to begin a descent. 

Fly into high density airspace an efficiency is flung out the window to meet controller’s/traffic needs. As Captain Megginson observes, weather and passenger comfort can take precedence – both in accelerating or reducing rates of descent to lower altitudes.

However, the biggest surprise in his response were the observations about “the old advice about saving fuel by using a faster cruise speed when flying against a headwind.” 

This was data that was taught to me as a military pilot and reiterated for many years in civilian flying – and it’s all wrong. Reader Megginson is completely correct when he says flying slower is more fuel efficient (to a point) not only in a tailwind – but also in a headwind.

Again he is bang on by claiming that we will save fuel by flying in that headwind at a reduced airspeed. But wait! Fuel isn’t the only cost and efficiency consideration related to flying costs. 

Time is another major cost factor and in fact, fuel flow is rather less important. Time may also be an expensive factor in a non-monetary sense as most of us fly to save time. Flying into a headwind at a reduced power setting may well save a gallon or so an hour – but not time.

There are many costs associated with the hourly operation of an aircraft that affect the outcome when considering the true price tag on flying an hour – or a mile. These include: oil changes, propeller overhaul, engine overhaul, scheduled maintenance plus wear and tear that might require unscheduled repairs not to mention loss in resale value with higher time aircraft. 

So, what are the numbers? Like Megginson, I started making calculations from my POH for the Cessna 172 and for some variability my POH for the Piper Cheyenne twin turboprop. 

An example for a Cessna 172 might realistically provide total hourly operating costs of say $80 and for the Cheyenne $300 an hour. (Readers should feel free to work out their own numbers for their aircraft). 

Let’s have a look at the true costs (and savings) of dealing with the effects of convection and the Coriolis effect (wind).

CESSNA 172 SAMPLE FLIGHT

Assumptions: Cessna 172 R, (the only Skyhawk POH I had), 160 hp Lycoming, cruising at 8,000 feet at gross weight of 2,450 pounds. Fuel cost $5/US gallon, hourly cost $80 and a 300 nm flight segment. (Adjustments and calculations were not made for start, taxi, climb and descent as they are not considerations for the studying of wind effects on cruise).

No wind,  Max cruise 81% power:

Flight time 2.52 hours, fuel used 21.93 gallons

Fuel cost $109.66 and airframe cost $201.60

Total flight cost $311.26

Thirty knot tailwind, 52% power:

Flight time 2.42 hours, fuel used 14.51 gallons

Fuel cost $72.55 and airframe cost $192.80

Total flight cost $265.35

Thirty knot tailwind, Max cruise 81% power:

Flight time 2.01 hours, fuel used 17.5 gallons

Fuel cost $87.58 and airframe cost $160.8

Total flight cost $248.38

Thirty knot headwind, 81% power

Flight time 3.37 hours, fuel used 29.32 gallons

Fuel cost $146.63 and airframe cost $269.60

Total flight cost $416.23

Thirty knot headwind, 52% power

Flight time 4.69 hours, fuel used 28.12 gallons

Fuel cost $140.66 and airframe cost $375.20

Total flight cost $515.86

PIPER CHEYENNE TWIN TURBOPROP

For the turboprop, I selected a typical average cruising altitude of 18,000 feet with minimum and maximum fuel flows of 324-730 pounds per hour and TAS at these settings from 207-328 mph. 

The data supported the same conclusions from the Cessna Skyhawk analysis above so I won’t belabor the point with readers. Nonetheless readers can use the preceding data for the P-31T on various ranges and wind speeds for their own edification.

CRUISE COST CONCLUSIONS

As can be readily concluded by the 172 data, a honking great tailwind when combined with a reduced power setting provides the optimum fuel efficiency for the aircraft operator but not the cheapest flight when the total costs are considered. 

When one factors in the non-fuel related costs of operation, flying at reduced power in a tailwind is more costly. When it comes to headwinds the most efficient way to fly the 300 miles is at the highest allowable power setting. While it costs $5 more in fuel, it saves a $100 in other costs.

Once again, thanks to David Megginson for taking the time to write, thereby triggering this more in-depth look at the true costs of operating in a those horizontal rivers of air.

Ken is a former COPA director who lives in Victoria, BC. He provides services internationally in advanced training, expert witness, flight test and aircraft sales. He has logged 15,000 hours on 375 types of fixed wing and rotary aircraft. Soaring his Diamond Xtreme is what he does for pleasure.