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Rules of Thumb
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Article Information | ||
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Category: | Theory of Flight | |
Content source: | SKYbrary | |
Content control: | Air Pilots |
Definition
Used correctly, rules of thumb (sometimes know as “heuristics") can assist significantly in pilot decision making and understanding. A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating some value. It is particularly useful as a means of cross-checking or confirming the validity of information being displayed by aircraft navigation systems and flight management systems.
1 in 60 Rule
A 1 degree offset angle at 60 nm equates to 1 nm of displacement.
Distance off track = (number of degrees off course x distance to station)/60
Speed/Distance/Time
Flying speeds that simplify mental arithmetic can help you in many ways, such as keeping retaining situational awareness during radar vectoring.
- 120Kts = 2nms/min
- 180Kts = 3nms/min
- 240Kts = 4nms/min
- 300Kts = 5nms/min
The 1 in 60 rule combined with Speed/Distance/Time assumptions is the basis of many other ‘rules of thumb’ that can be useful in pilot navigation (or to check that an FMS-calculated track makes sence. For example:
At 120 kt groundspeed, the aircraft travels 60nm in 30 minutes. A 10 kt wind blows the aircraft 5 nm in 30 minutes At 120 kt groundspeed, a 10 kt crosswind will cause 5 degrees of drift
Maximum drift angle (Max Drift) = Windspeed divided by Groundspeed in miles per minute
Conversions
1 m/s = 2 its = 4 km/hr approx
Crosswind Component
Useful for evaluating runway crosswind from reported wind, the crosswind is a function of the SINE of the angle between the runway and the wind direction. Therefore, crosswind can be estimated as follows:
Angle between wind
and runway (degrees) |
Crosswind component
(% of wind strength) |
Sine of angle between
wind direction and runway in degrees |
0 | 0 | 0 |
15 | 25 | 0.26 |
30 | 50 | 0.5 |
60 or more | 100 | 0.87 |
The analogue clock face provides an easy way to remember this:
15 min = ¼ of an hour 15 degrees off = ¼ of the total wind across 30 min = ½ of an hour 30 degrees off = ½ of the total wind across 60 min = A full hour 60 degrees off = All of the wind across
A similar process can be used to estimate wind effect on groundspeed
Combining Max drift and Crosswind component:
- Flying at 420 kt groundspeed (7 nm/min) in the vicinity of a 60 kt wind (approx. 8½ degrees max drift) headwind from 30 degrees off track, the expected drift angle is just over 4 degrees.
- Flying at 420 kt airspeed in the vicinity of a 60 kt wind from 30 degrees off track, groundspeed will be approximately 360 kt (~6 nm/min) so max drift is 10 degrees and the expected drift angle will be 5 degrees.
Slant Range
Overhead a Distance Measuring Equipment (DME) the indicated range will be equal to the altitude of the aircraft. One NM is approximately 6,000’ (actually 6,076’)
Slant Range Overhead DME = Altitude in feet/6000
Horizon Range
The horizon (in nautical miles) will be approximately the square root of the height in feet:
- At 10,000ft, the horizon at at approximately 100nm
- At 20,000ft, the horizon at at approximately 140nm
- At 30,000ft, the horizon at at approximately 170nm
Descent Range
Different types will have different performance so pilots must establish and check any ‘rule’ for their own aircraft.
- 30 per 10 plus 10…
For many older jet transports, a normal descent from cruise altitude descent required about 30 nm for each 10,000ft of height loss and a further 10 nm to slow down. Therefore:
- 30,000’ cruise = (3 x 30) + 10 = 100 nm descent
- 35,000’ cruise = (3.5 x 30) + 10 = 115 nm descent
Although not strictly accurate, it provided a good first guesstimate.
Modern, more efficient aircraft, will need greater distances but similar rules of thumb can often be defined from a review of performance figures and line experience. You may find that (e.g.) “40 per 10 plus 15” works better for your type. The important point here is that well practiced rules of thumb may need to be revised dramatically when changing from one type to another.
Similarly, to confirm that a descent profile is going well:
- 30 out at 10 and 250….
Thirty miles from the airport at 10,000' and 250 knots.
If at 30 nm from destination, the aircraft is still above either 10,000’ or 250 kt (or both!) getting down and reducing speed to achieve a stabilized approach will be a real challenge in many jet transports.
Diagram showing Descent range rules of thumb
Final Approach
- 3 degree glideslope = 300’/nm to touchdown
Again from the 1 in 60 rule, 3 degrees at 60 nm ~ 3 nm ~ 18,000’ so 3 degrees at 6 nm ~ 1,800’ and 3 degrees at 1 nm ~ 300’)
This is not exact – and approach plates will show precise figures for any approach - but it provides a simple way to spot any gross errors.
Rate of Descent on Final Approach
For a 3 degree glideslope, required rate of descent in feet per minute is approximately equal to ground speed in knots multiplied by 5.
From the above, at 120 knots GS, the rate of descent to maintain a 3 degree glideslope is approximately 600 fpm
Payload and Fuel
It is always useful to check mentally that loading figures make sense. While it may not be true for every situation (so pilots must review the circumstance of their own operation before using this), many pilots find the following rule of thumb effective:
- 10 pax equates to 1 ton.
For a very rough estimate: Trip fuel = flight time x cruise fuel flow
ILS
At the threshold, 1/2 LOC dot = 1/2 runway width
On a very foggy take-off if you think you are lined up on the C/`L lights and you see half a dot deviation on the ILS, you must be looking at the edge lights!
Weather Radar
You can use the 1 in 60 rule to determine height of weather returns. However, remember the beam width is typically +/- 2 deg.
You can measure beam width in flight by looking for the range of the (first) ground return from altitude for a given search angle, e.g.: At 30,000ft, tilt -1 deg, if first ground return is 100 miles, beam width is +/-2 deg (lower edge of the beam is tilted -3 degrees).
If you have any other rules of thumb that you find useful then please send the information to the Editor