Here’s a quick rule of thumb for quickly converting pounds of fuel to gallons: Take fuel load in pounds, drop the last digit, divide by 2, and add the result to the original amount in pounds minus the last digit. For example, 1,000 pounds of fuel, drop the last digit to get 100, divide by 2 to get 50, and add 50 to 100 to get 150 gallons equals 1,000 pounds of fuel.
To covert gallons of fuel to pounds divide the number of gallons by 3, subtract the result from the original number and add a 0. For example, 150 gallons divided by 3 equals 50, subtracted from 150 equals 100, add a 0 and you get 1000 pounds of fuel.
Now let’s look at handy rules of thumb for planning profiled descents, and calculating rates of descent on an ILS. Remember, however, that the most accurate information is printed in the aircraft Operator’s Handbook.
STARTING A DESCENT
A quick-reference rule of thumb that can assist us on a daily basis is the rule used to calculate the beginning point of a descent out of altitude (top of descent—TOD), and its associated rule used to calculate the rate of descent. The axiom is:
- Three times the altitude to lose equals top of descent in nautical miles.
- Five times the ground speed is equal to the rate of descent on a 3-degree descent profile.
Use of this rule allows us to profile our descents, thereby preventing early or late descents leading to missed crossing restrictions or excessive fuel burns caused by long cruise segments at low altitudes. This rule may appear to be laborious, but with practice it can become second nature. One part of this rule that I particularly like is that it uses a 3-degree-descent profile. Let’s examine the use of this rule in a real-world example.
We are in cruise, level at FL 270, 128 miles west of Omaha, with a ground speed of 280 knots. ATC issues the following clearance: “Commander One-Two-Three Fox Sierra, at pilot’s discretion descend so as to cross fifteen west of Omaha at and maintain ten thousand feet.” Now we must calculate when to start down to ensure that we make the crossing restriction, and then calculate a descent profile that will get us to the assigned altitude at the fix. Here we go.
Three times the altitude to lose (divided by 1000) equals the TOD in nautical miles. In this case we need to lose 17,000 feet. Three times 17 equals 51, so 51 miles prior to the fix we should begin our descent. In this example the crossing restriction is 15 miles west of Omaha so we need to include this in our calculations if we are using the OMA DME as our reference. That makes our TOD point 68 miles west of OMA.
DESCENT RATE
Five times the ground speed equals the rate of descent. In this example we would need an initial descent rate of 1400 FPM. For those of you grabbing for the E6-B, I know that if we flew this profile as calculated we would be about 1,800 feet high when we arrived at the fix, thereby irritating the ATC specialist and potentially subjecting our skills and abilities to some FAA scrutiny. Remember, that’s why these are called rules of thumb; consistent updates are needed to increase accuracy.
Ground speed will vary as we descend so we need to monitor the descent and vary the rate of descent to remain in accord with the ground speed. The good part about this axiom is that it allows you to continually monitor the descent and adjust the descent rate to arrive at the fix at the proper altitude simply by making a few additional calculations.
For example, when we get to a point 51 miles from OMA (36 miles from the crossing restriction) where should we be? Divide the distance remaining (36) by 3 and that will equal the height that you should be above the crossing altitude in thousands of feet.
In this example 12 plus 10 (the crossing altitude) shows that we should be at FL 220. You can also work it the other way. With 12,000 feet to lose we should be 36 miles from the fix. If not, we can adjust the rate of descent until we are back on schedule.
When we arrive at the 40 DME fix we should be descending through 18,300 feet. At 15,000 feet we should be at the 30 DME fix. Once you learn to work with this little rule of thumb you may find that your descents are much easier, you will worry less about crossing restrictions or entering the pattern at the correct altitude, and you will not spend a lot of time at low-altitude cruise burning excessive amounts of fuel.
WIND COMPONENTS
One other axiom that has seen a lot of use is the calculation of headwind/tailwind and crosswind components using the sine and the cosine of an angle. Use the chart below to try this for yourself.
Wind Component Table
| Wind Angle | Crosswind | Headwind |
| Sine | Cosine | |
| 0.00 | 0.00 | 1.00 |
| 10.00 | 0.20 | 1.00 |
| 20.00 | 0.35 | 0.90 |
| 30.00 | 0.50 | 0.85 |
| 40.00 | 0.65 | 0.75 |
| 50.00 | 0.75 | 0.65 |
| 60.00 | 0.85 | 0.50 |
| 70.00 | 0.90 | 0.35 |
| 80.00 | 1.00 | 0.20 |
| 90.00 | 1.00 | 0.00 |
The headwind/tailwind component of any wind is equal to the cosine of the wind angle times the wind velocity. The 90-degree crosswind forces are equal to the sine of the wind angle times the wind velocity.
A 40-knot wind 50 degrees off the tail would give us a 26-knot tailwind component and a 30-knot crosswind component. This ROT might come in handy when you are trying to decide whether or not you should try to land in that 50-knot wind that is 45 degrees to the runway.
ILS RATE OF DESCENT
One final rule of thumb is used to calculate what your rate of descent should be on an ILS glide slope. This equation is based on a 3-degree glide slope, and it is fairly accurate. The rate of descent for a 3-degree glide slope is the groundspeed divided by 2 with a 0 added. As an example, we should be descending at 600 fpm on a 3-degree glide slope with a groundspeed of 120 knots. If we refer to a descent rate table we would find that 635 fpm is actually required to maintain the glide slope, but for planning purposes the rule of thumb is a good tool.
Remember, rules of thumb are approximate and should be used with that in mind. With practice and updates, a great deal of accuracy can be achieved.
Good Flying!
