Three Useful Aviation Rules of Thumb
Rule of Thumb #1 – Rate of Descent to Remain on Glideslope
One of the most useful rules of thumb I’ve relied on throughout my helicopter flying career—is estimating the rate of descent required to maintain a three-degree glidepath.
Multiply your groundspeed by five, and you’ll have a close approximation of the required descent rate.
Examples:
- 90 knots groundspeed → ~450 fpm (actual = 478 fpm)
- 120 knots groundspeed → ~600 fpm (actual = 637 fpm)
- 60 knots groundspeed → ~300 fpm (actual = 318 fpm)
This target descent rate is helpful on any approach with vertical guidance displayed (ILS, APV, VNV, etc.), but it becomes even more valuable during precision radar approaches (PAR), where you do not have a visible glidepath indicator to follow.
I’ve also used this rule of thumb to validate descent rates during instrument approaches to a hover—procedures used in maritime SAR to establish a stable hover in hard IMC. In the United States Coast Guard, this technique is referred to as a MATCH, and I referenced this rule of thumb at the end of this post.
Rule of Thumb #2 – Pounds of Fuel per Mile
In the MH60T—the aircraft I’ve flown most throughout my career—max range airspeed was typically around 125 knots at sea level for our operating weights. At that speed, fuel flow was roughly 1,200 pounds per hour.
That combination conveniently works out to a slightly conservative burn rate of about 10 pounds per nautical mile, which gave me a quick rule of thumb to calculate fuel consumption: take the distance and add a zero. For example, 135 nautical miles ≈ 1,350 pounds of fuel.
On time-sensitive launches or diversions, this simple estimate provided immediate situational awareness of expected fuel on arrival—allowing me to quickly assess on-scene time, bingo fuel, and overall mission feasibility.
While the math isn’t always this clean in other aircraft, understanding a rough pounds-per-mile burn rate remains a valuable tool for rapid decision-making and double checking for data entry errors when using computer derived fuel planning.
Rule of Thumb #3 – True Airspeed and Max Range at Altitude
Most maximum range charts provide the max range indicated airspeed (IAS) for a given altitude and weight. As altitude increases, IAS for max range decreases. Referencing IAS makes sense, since IAS is displayed in the aircraft.
However, when planning how far you can actually travel, indicated airspeed isn’t enough. You need true airspeed (TAS), which—combined with winds—determines your groundspeed and overall range.
A useful rule of thumb:
TAS ≈ IAS + 2 knots per 1,000 feet
For example, on a long-range mission at 8,000 feet, if your max range speed is 100 KIAS, your TAS is approximately 116 knots. In a no-wind scenario, that means you’re flying 100 KIAS but covering about 116 nautical miles per hour over the ground.
This rule of thumb assumes a standard day and standard lapse rate. Significant temperature deviations will affect true airspeed—warmer-than-standard conditions increases TAS compared to IAS—colder-than-standard conditions decreases true airspeed compared to IAS.
More MH60T max range considerations can be found in these posts:
Max Range Considerations – SPIRAL UP
Max Range Calculations/Planning (Continued) – SPIRAL UP
Even in the era of GPS, glass cockpits, and CDU-derived performance planning, rules of thumb remain valuable. At a minimum, they provide a quick cross-check—helping pilots catch automation or data entry errors before they become problems. Garbage in still leads to garbage out. More rules of thumb to follow.