## The Nav Log for the Short Cross Country – Part 1

It’s time for my cross countries. One of the things needed for a cross country is the nav log. The nav log is essentially a spreadsheet that allows you to calculate how long you will be in the air during the cross country. Knowing how long you’ll be up let’s you determine how much fuel you need and when you will be arriving at specific points in your journey.

You need to calculate your fuel for a few reasons. First, of course, you want to ensure that you have enough for the trip. If you know how much you need you can, if require, carry less fuel. This can help you with your weight and balance. If you need full tanks to complete your journey but you’ve got too much weight on board to allow full tanks the fuel calculation and the weight and balance calculation will indicate this and you can plan for a mid-route fuel stop.

You need to calculate time for a few reasons as well. First, if you want to file a flight plan you need to know when you can expect to land. Also, the amount of time aloft is what allows you to calculate your fuel requirements. Navigation checks depend on time as well – you can choose some pre-determined checkpoints on the map, calculate how long it should take you to reach them, and then check to see if you’re on course by comparing when you reach the checkpoints with your estimate of time. Of course, if you never reach the checkpoint you’ll know that you’re off course.

Step one ins the nav log is the route. The shortest nav log possible will have the starting point, a set heading point (SHP), the top of descent (TOD), and the end point. This assumes either a straight line between the take off and landing points, or change of headings only at the SHP and TOD.

As an example of a short cross country I am using the route from Pitt Meadows (CYPK) to Chilliwack (CYCW). Chilliwack is pretty much straight east of CYPK. Usually I take off from CYPK to the west, so it’s exactly the opposite direction of where I want to go. However, probably 25% of the time I take off to the east. Sometimes I take off of the north or the south. I won’t know which runway is active until the day of the trip, so I’ve got a problem in terms of nailing down my starting point.

This is always the case, actually. The active runway changes because of weather. Also, the destination changes from trip to trip. The actual starting point for the trip is always a challenge to nail down. The solution is something called a set heading point (SHP). The SHP is a place on the map where we will be at cruise altitude and cruise speed. It’s also a place that will be easy to identify from the air.

Heading west from CYPK a lot of people choose the end of the Port Moody inlet. It gives you enough time to make the altitude you want, get into cruise attitude and is easy to identify. Heading east, toward CYCW, I’m going to choose the western tip of the island in the Fraser just before Fort Langley to the south and Albion to the north. That will be the entry in the first cell of my “From/To” column, and I’ll write it in as “CYPK/SHP”.

The next leg of this cross country will be a straight line from the SHP to the TOD. All one heading, and all one altitude. This goes in the second cell in the “From/To” column. It is written “SHP/TOD”.

The last leg of this cross country is the TOD to CYCW. This goes in the third cell of the first column. It is written in as “TOD/CYCW”. For this short cross country that completes the first column.

The second column can be filled in now. It is the altitude column, and is often labelled “ALT (ASL)”. The altitude from take off at CYPK to the set heading point will be changing because we’re climbing and leveling out. The starting point changes based on the active runway. The active changes based on weather. Weather just changes. Therefore, we can’t determine with exactitude what our altitude will be during the first leg. We will instead use an educated assumption. What we write in the first cell of the “ALT” column, however, is a straight line arrow starting in the lower left corner and pointing at the upper right corner. The second cell in the “ALT” column is the altitude we’re going to use between the SHP and TOD. I’m choosing 2,500 ft. ASL, and that’s what I write in the nav log. The choice of altitude is determined by route and weather and often changes. The third cell in the “ALT” column deals with the leg from the TOD to the runway at CYCW. It gets a diagonal line from top left to bottom right. Circuit height at CYCW is 1000 ASL, so 1000 goes into the lower left half of the cell.

On the sheet I use the next column is pressure altitude and temperature. This column is labelled “P.ALT/TEMP”. Pressure altitude is important as a starting point for further calculations. The altimeter is set for standard pressure at sea level. Changes in pressure require changes to the altimeter, which is why we check and set it during the initial phase of any flight. For navigation purposes pressure altitude matters because the air at our choosen altitude will be either thinner or denser than standard (unless we’re actually at standard pressure). This will effect fuel burn and speed. You need more fuel to fly through denser air and less fuel to fly through thinner air, assuming a constant speed. More to the point, if we keep altitude and RPM constant then speed will vary based on the air pressure.

If you want to figure out pressure altitude yourself, either on paper or with a calculator or computer spreadsheet, the equation is simple. Take standard pressure of 29.92, subtract the altimeter setting for that day, and then multiply the result by 1000 and add that to your planned cruise altitude. A higher pressure day will result in adding a negative number to the altitude (i.e., you’ll be subtracting) while a lower pressure day will add to P.Alt. Assume that the pressure for the day of the trip is 30.12. 29.92 – 30.12 = -.20. If you multiply -.20 by 1000 you get -200. Adding -200 to my choosen altitude of 2500 gives me a pressure altitude of 2300. This gets written into the top of the second cell in the third column “P.ALT/TEMP”.

The bottom half of the second cell in the “P.ALT/TEMP” column is the temperature for the day of the flight. METAR temperatures are taken at the surface, but I’m flying at 2500 feet. Normal adiabatic temperature lapse rate is 2 degrees per 1000 feet, so if the METAR tells me 8 degrees I’ll write in 3.5 in the space.

The next column is RPM and true airspeed. It is labelled “RPM/TAS”. I fly a Cessna 150L, and the POH says that at 2500 feet and 2400 RPM I’m flying at 60% brake horsepower with a true airspeed of 103 mph and a fuel burn of 4.6 gallons per hour. I’m happy with that RPM setting, and the other numbers just follow from that. I convert the TAS into knots and get 89.5 knots (.868976242 knots per mile). I write in 2400 in the upper half of the cell and 90 in the lower. I can also go to the far right of the nav log and write in the fuel burn rate in the second and third cells for the “FUEL-Gal/Hr” column.

So far this has been pretty straightforward. All calculations have been very simple, and constant enough to put in a simple spreadsheet. The next step requires that I convert TAS to calibrated airspeed (CAS) and then convert that to indicated airspeed (IAS). This requires use of the E6B.

The first number I need is the pressure altitude. I know from the third column calculations that my pressure altitude is 2300. I also know, from the same place, that the temperature will be 5.5 degrees. On the E6B there is a temperature range reading from +50 on the left to -50 on the right. This is found right under the black label reading “Density Altitude”. The temperature I’m using is +5.5 centigrade. The marks are too small to get half degrees, so I use 5 degrees and line it up with 2300 feet underneath it. Without moving the wheel I then read 90 knots on the outside wheel (that’s where TAS numbers are) and right across from that is the CAS. 90 TAS at 2300 ASL at 5 degrees = 88 knots CAS.

The POH for my plane has a correction table for CAS/IAS. It tells me that for a CAS of 88 knots the IAS is 90 knots (you have to read between the lines). That number can go in the second and third cells of the fifth column (the one labelled “IAS”).

The next column, True Track, is simple to fill in. For the first leg, CPYK to SHP, I write in “Vis”, because all I’m going to do is take off and fly the pattern until I leave it to fly directly to my SHP. From my SHP I have to fly a specific heading to the TOD. That heading comes from my chart, and in this case its 93 – like I said, CYCW is almost directly east of CYPK. The second cell in the 6th column, therefore, will be 93. The last leg is from TOD to CYCW circuit, and again, it’s marked in as “Vis”.

True track is the track on paper that will take me to my destination. That track gets modified by wind as well as by east and west magnetic variance. If I can determine what those modifications are I’ll know what actual heading I have to fly to accomplish the true track on paper. The first variable is wind, so I have to check the weather. For the last flight I did on this route the winds were from 260 at 16 kts. This information goes into the E6B to find out the wind correction angle. Use the opposite side of the E6B for this – the side with the compass wheel.

Put the grommet over the 100 mark on the center line. Put the wind direction (260) under the True Index mark on the E6B. Measure up from the grommet the velocity of the wind (16 kts) and make a mark (use a lead pencil and mark a cross).

Spin the wheel until the true track heading (93) is under the True Index mark. The wind velocity mark has spun as well, and is no longer on the center line. Slide the wind velocity mark up until it coincides with TAS (90 kts). Ground speed will read under the grommet and the Wind Correction Angle (WCA) will read between where the wind velocity mark is and the center line. In this case I’m flying east at 90 kts. The wind is coming from the west, so it’s a tail wind. The E6B says my ground speed is going to be 106 kts, and my WCA will be 1 degree. The wind velocity mark that gives me the WCA is on the right hand side of the center line, which is the west. The mnemonic is “west is best, east is least” so the 1 degree WCA is a plus (if it had been on the east dies of the line I’d subtract it.

This calculation lets me fill two columns. The first is column 8, wind correction angle, labelled “WCA”. The second column is the 12th column, ground speed, labelled “G/S (kts)” I enter “+1″ in the WCA column and “106” in the ground speed column. I can enter 94 in the True Heading column (labelled “True HDG”).

Magnetic variation comes right off the chart. For this flight it’s -19. That figure goes into the second cell of column 10. Now I’m ready to do the math to get my magnetic heading, which is the one that I’ll fly based on the heading indicator. I start with 93 (true track), add 1 degree for wind correction (it’s almost a direct tail wind, so it’s a small correction). That gets me to 94 degrees. I subtract 19 for magnetic variation and arrive at 73 degrees as the magnetic heading. This gets entered into the second cell of the 11th column.

All the columns from 1 to 12 are now filled. Column 13 is the distance column. On the first leg I climb, reach cruise altitude, and fly to my SHP. The total distance is 7 nautical miles, but only 4.5 of that will be in the climb. The balance, 2.5, will be at cruise. The E6B comes in again, although a spreadsheet can also give you the answer. The math is simple if we have the numbers. For the climb we’re going to assume our ground speed is 64 kts. The E6B tells me that I’ll need 4.25 minutes. The math is 64 nautical miles in 60 minutes is the same as 4.5 nautical miles in x minutes. 4.5*60/64= 4.218 minutes. The balance of the distance to the SHP is 2.5 miles, but we’ll be flying at a ground speed approaching 106 kts. 2.5*60/106= 1.41 minutes. The E6B gives an answer of 1.35 minutes. I’ll round both numbers up and use 5 and 2 for 7 total time.

The next leg distance is 21 miles. The E6B says just shy of 12 minutes. Math says 21*60/106=11.88. The last leg, from TOD to CYCW, measures 4.5 miles at 106 kts. E6B says that will take about 2.5 minutes. Math says 4.5*60/106= 2.547 minutes.

The last column to figure out now is fuel burn. From take off to SHP will take 1.4 gallons. 2 minutes to SHP at 4.6 gph is 4.6/60*2= 0.15. I’ll put in .5 to be on the safe side. The rest is math. Don’t forget the reserve.

My name is Rob Chipman and I’m a realtor and pilot based in Vancouver, BC. I AM NOT A FLIGHT INSTRUCTOR AND I AM NOT OFFERING FLIGHT INSTRUCTION! I am sharing my study notes and other things I’ve learned while getting my education as a pilot. You’re welcome to make use of this information, but do not treat it as expert advice.

I really enjoy flying, real estate and the Chilcotin. My company is Coronet Realty Ltd., located at 3582 East Hastings Street, Vancouver, BC, V5K 2A7. I have a C-150L that I own with two other pilots, based out of Pitt Meadows. Do not hesitate to contact me by email if I can help you do anything, especially if its likely to be interesting or concerns selling remote property in British Columbia.