Saturday 3 September 2011

As the crow flies...

Now that we've added powerful GoogleMaps functionality to iNavCalc (see recent post), here's a very simple quick-start tutorial which demonstrates the most basic iNavCalc map features on the simplest of navigation applications: namely to find the distance "as the crow flies" between two locations anywhere in the world .

( well as a serving as a very basic intro to iNavCalc maps, this demonstration is handy in it's own right since one is often faced with the question "how far is it from so-and-so to so-and-so" -- now you can get the answer quickly via any browser on any device for any locations in the world...)

Monday 22 August 2011

Announcing iNavCalc Google map interface

We are pleased to announce a significant new enhancement to iNavCalc functionality: the ability to create and edit routes and waypoints using the convenience of Google maps. To get started right away, click here for layout optimised for desktops, and here for layout optimised for tablets (iPADs). No registration is required for access to the basic functionality. Registration required for access to full suite of features. Registration is FREE.

Key features of iNavCalc map interface:

  • Built upon Google maps with all the functionality that they offer such as global coverage, satellite photo overlays, etc.
  • Optional display of airfields, navaids, and user waypoints from the FlyLogical database. Selectable on a country-by-country basis to avoid clutter and to minimise browser load time.
  • Powerful search widget for searching the FlyLogical waypoints database as well as the global Google database. If no match found in the FlyLogical database, automatically performs a global Google GeoSearch. With this combination, you can find virtually any location in the world and use it as a waypoint in your route.
  • "Add to Route" button for adding any waypoint to the route with a single click.
  • Clickable map,  enabling any point to be selected via the mouse cursor (or touchscreen on tablet) by simply clicking on the map.
  • Latitude and longitude coordinate editors in a variety of commonly-used formats, enabling the map cursor to be precisely located (and a "move to" function enabling any waypoint to be "nudged" to the location of the cursor.
  • New waypoint can be added to the end of the current route, or can be inserted in any position along the route.
  • "Rubber banding" functionality enabling any waypoint to be clicked-and-dragged to any position (or precisely re-positioned via the lat/long input editors)
  • Toggle zoom function (convenient for quickly visualizing an airfield, then zooming back out)
  • Magnetic variation and sunrise/sunset times automatically computed for any point selected on the map.
  • "Quick plan" (assuming zero wind) calculations (distance, time, fuel) automatically computed and displayed in a banner at the top of the page whenever the route is modified. Parameters for the "Quick plan" (speed, fuel consumption, start fuel) are fully-editable (units are set via the iNavCalc Culture parameter, requires registration which is FREE)
  • "Traffic light" feature which automatically changes the colour of the quick plan banner depending on fuel consumed on route (green for less than 80% available consumed, amber for between 80% and 90% available consumed, and red for greater than 90% consumed). Provides an at-a-glance view of the fuel-plan for the route.
  • "Radials" feature for determining and specifying locations on the map based on radials from a specified centre-point position. Any number of radials emanating from any number of centre-points can be included on the route map. Radials can be specified as either "Magnetic" or "True". If "Magnetic", the magnetic variation of the given centre-point is automatically determined when computing the radial. For convenience, all radials are saved along with the route, for later re-use (note: route-saving functionality requires registration which is FREE).
  • Full integration with iNavCalc's comprehensive flight navigation planning functionality including AutoMETic. Simply click on "Get PLOG by email" to generate a comprehensive flight navigation plan (PLOG) including automatic computation of wind and temperature effects based on automated global weather reports and forecasts.  This provides a more detailed and accurate plan compared with the at-a-glance "Quick plan".  
  • Full integration with iNavCalc's Route and Waypoints Managers for saving routes and waypoints to the FlyLogical database for later re-use (requires registration which is FREE).

Saturday 16 July 2011

Announcing iBulldog

Our newest free app, iBulldog, is now live. This is a niche application for owners/operators of Scottish Aviation Bulldog aircraft, enabling you to calculate the Fatigue Index ('FI') for your airframe.

The Fatigue Index ('FI') determines the available life remaining in the Bulldog airframe. It is calculated from measurements of acceleration obtained from the 'Fatigue Meter' instrument, recorded after each flight. In the UK, for continued air-worthiness under CAA rules, each Bulldog owner/operator is obliged to send the readings each year to DeHavilland Support Limited (DHSL) who compute the revised life-remaining, according to formulae devised by British Aerospace (formerly, Scottish Aviation) during the full-scale fatigue life tests conducted as part of the original certification of the aircraft in the 1970s. 

Our iBulldog app gives you the ability to assess the Fatigue Index at any time within a given year, i.e., before the next official evaluation by DHSL. The results are accurate to within 4 decimal places of the DHSL  calculations. By enabling you to predict (approximate) FI consumption (for a given set of assumed FI readings), the app helps you to assess, a priori, the relative effects on lifetime of, for example, aerobatics versus circuits & landings, or steep-turns/air-racing versus gentle aeros. Armed with this information, you can fine-tune your flying profile accordingly.

Sunday 19 June 2011

When the wind blows, you always lose on the roundtrip

Update 18 April 2017: completely re-written iNavCalc mobile apps with streamlined command-line interface for iOS, Android, and Windows 10

You sometimes hear pilots claiming "I don't mind the headwind on the way out, because the tailwind on the way back will 'blow me all the way home' and make up for it". You probably know this to be untrue, but I thought it would be useful to prove that this is wholly untrue. With some very simple maths, we can quickly see that whenever the wind blows, you always lose out in round-trip time and fuel consumed. Here's why:

First, some simplifying assumptions to make the points clear:
  • TAS (true airspeed) is constant along the route, denoted V
  • Windspeed is in constant direction with constant speed, denoted W
  • Rate of fuel consumption, denoted R, is constant along the route
  • Outbound leg is directly into wind. Groundspeed outbound is therefore V-W
  • Return leg is directly downwind. Groundspeed inbound is therefore V+W
With these simple assumptions, and for a leg distance denoted D (each way), we arrive at the following results for time en-route, denoted T, and fuel consumed, denoted F, for each leg:

Time (T)Fuel  (F)Comments
D/(V-W)D*R/(V-W)As expected, the effect of the headwind is to reduce the magnitude of the denominator, thereby increasing the time and fuel consumed for the leg
D/(V+W)D*R/(V+W)By contrast, the effect of the tailwind is to increase the magnitude of the denominator, thereby decreasing the time and fuel consumed for the leg

It is instructive to define these results with respect to the difference with the corresponding values in zero-wind, which we'll denote by T'=D/V and F'=D*R/V respectively. The differences are denoted ΔT=T-T' and ΔF=F-F'. We thus obtain the following expressions for the relative (or fractional) differences compared with the zero-wind baseline, where we have used for convenience the symbol r=W/V (ratio of windspeed to TAS):

ΔT/T' (or equivalently ΔF/F') Comments
r/(1-r)The difference is positive, as expected for the headwind
-r/(1+r)By contrast, the difference is negative for the tailwind. However, and moreover, the denominator in the tailwind expression (1+r) is always numerically larger than in the headwind expression (1-r), for any r, which means that the advantage offered by the tailwind is always numerically smaller than the penalty from the headwind. The two effects never cancel-out: the headwind always wins, you always lose.

It is instructive to now consider the total round-trip by adding the differences for the outbound and return legs, and dividing by the zero-wind round-trip values (2*T' and 2*F' for time and fuel, respectively) to give the relative (fractional) difference in round-trip time and fuel as:

Penalty= (ΔT/2T') = (ΔF/2F') = r2/(1-r2)

...from which we can make the following observations:
  • The expression is always positive for any value of windspeed and TAS, which means you always lose out on the round-trip. This is the central result of this article.
  • As windspeed increases, the round-trip penalty increases by greater-and-greater amounts (due primarily to (1-r2) in the denominator. This means that the higher the windspeed, the greater the penalty.
  • The expression is only valid for values of r from 0 to 1. At r=0, this represents the zero-wind condition, and the expressions evaluates to 0, as expected.  At r=1, the windspeed equals the airspeed and the penalty is infinite since the outbound groundspeed is zero and the leg is never completed.
Let's now look at some typical realistic values:

expressed as %

V=100 kts; W=10 kts



Negligible penalty since 1% ought to be well within your planned fuel margin

V=100 kts; W=20 kts



Penalty more significant at 4%, but still ought to be within your planned fuel margin

V=100 kts; W=30 kts



Penalty significant at 10%. Starts to impact on fuel margin. Moreover, a 30 kt wind is not uncommon at GA altitudes.

V=100 kts; W=40 kts



Penalty significant at 19%. Serious impact on fuel margin unless distance is short. Probably should not fly into a 40 kt headwind in a light aircraft for any significant distance unless there is adequate fuel margin.

V=100 kts; W=50 kts



Penalty huge at 33%. However, it is highly unlikely you would fly into a 50 kt headwind in a light aircraft for any significant distance. That said, for an ultralight with a TAS of, say 70 kts, the ratio of r=0.5 is quite easily encountered i.e., with a headwind of 35 kts. Beware: you will require 44% more fuel for a round-trip under such conditions !

V=100 kts; W=70.7 kts



Out of fun/curiosity, you can work-back the expression to demonstrate that the penalty becomes 100% (i.e., you double your en-route time and fuel for r=(1/√2) or 0.707). Unlikely you would ever encounter such extreme headwind (unless you were in an airship, or such!).

We can now explore further variations on the theme using iNavCalc which performs all such "velocity triangle" calculations effortlessly. Normally, you would use iNavCalc to automatically calculate for the given actual/forecast weather along the planned route (we call this "AutoMETic"). However, you can optionally switch-off the automated weather functionality and specify the weather (winds aloft etc) manually. This is particularly useful for demonstration/educational/training purposes, and so we will make use of that capability now.

To get started, let's consider the idealized scenario presented earlier. Namely, a round-trip flight conducted with the outbound leg directly into wind, and the inbound leg directly downwind. Let's assume the following parameters for the calculations:

IAS (cruise) 96 kts This gives a TAS of 100 kts at 3000 ft altitude in ISA conditions
Altitude 3000 ft Typical altitude for GA VFR flight
Start fuel 150 litres Typical available fuel (full tanks) for light aircraft
Fuel consumption (cruise) 40 litres-per-hour Typical cruise fuel consumption for light aircraft
IAS (climb) 80 kts Typical climb IAS for light aircraft. Note: iNavCalc accounts for climb as well as cruise in the time and fuel calculations, so we need to specify the climb IAS.
Fuel consumption (climb) 40 litres-per-hour iNavCalc accounts for enhanced fuel consumption in climb, so we can specify accordingly.
IAS (descent) 96 kts Typical descent IAS for light aircraft. Note: iNavCalc accounts for descent as well as cruise in the time and fuel calculations, so we need to specify the descent IAS. In this example, we will specify same as for cruise.
Fuel consumption (descent) 40 litres-per-hour iNavCalc accounts for enhanced fuel consumption in descent, so we can specify accordingly.
In this example, we will specify same as for cruise.
Climb rate 1100 feet per minute Typical climb-rate for light aircraft. Required for time and distance calculations in climb.
Descent rate 600 feet per minute Typical descent-rate. Required for time and distance calculations in descent.
Sea level outside air temperature 15 C ISA conditions (note: iNavCalc would normally determine this automatically. However, we are switching-off AutoMETic for now, so we need to specify).
Sea level atmospheric pressure 1013.25 mb ISA conditions (note: iNavCalc would normally determine this automatically. However, we are switching-off AutoMETic for now, so we need to specify).

To specify the route, let's choose an arbitrary starting point given by the following coordinates: 540500N 0043724W (this happens to be Ronaldsway Airport, Isle of Man ICAO: EGNS). We will fly due (true-)north for 100 nm, then due (true-)south for 100 nm, completing the round-trip.

Let's start with the baseline (zero-wind) case. In which case, we will set the windspeed to zero, and the complete iNavCalc parameter-string, encapsulating all of the foregoing, is given by:

route={540500N 0043724W MyStart}, >360/100, >180/100; bearingtype=true; culture=UK; startfuel=150; ias=96; alt=3000; fuelflow=40; iasclimb=80; iasdescent=96; fuelflowclimb=65; fuelflowdescent=40; climbrate=1100; descentrate=600; met=manual; oat=15; qnh=1013.25; wind=360/0

(Click here for full documentation describing all  iNavCalc parameters and their usage.)

All we need to do now is send an email to with the above parameter-string as the subject-line. The resulting response email  contains a comprehensive navigation planning log (PLOG) for the flight. From this, we can pick out the desired values for present purposes. Namely:

  • Total distance: 200 nm
  • Total flight time: 2hrs 0 mins
  • Total fuel consumed: 81.2 litres

These represent the baseline (zero-wind) values for the round-trip.

We can now add non-zero wind in the northerly direction (direct headwind). For example, for the windspeed of 10 kts, we simply specify the following parameter-string:

route={540500N 0043724W MyStart}, >360/100, >180/100;  ias=96; alt=3000; met=manual; oat=15; qnh=1013.25; wind=360/10

(Note: we are taking advantage of the "sticky" parameter functionality whereby we do not need to specify all of them again if they haven't changed. Click here for full documentation describing all  iNavCalc parameters and their usage.)

The resulting PLOG reveals:

  • Total flight time: 2hrs 1 mins (0.83% penalty, compared with 1% from earlier analytical expression for r=0.1)
  • Total fuel consumed: 82 litres (0.98%  penalty, compared with 1% from earlier analytical expression for r=0.1)
...which are in close agreement with the results of the earlier analytical calculation (i.e., 1% penalty for r=0.1). The differences are due to the fact that  iNavCalc takes careful account of climb and descent performance (on time and fuel calculations) as well as cruise, whereas the analytical calculations pertain to idealized cruise conditions.

We can continue with the exercise by putting successively higher windspeed values into  iNavCalc, and recording the results obtained. They are summarised below alongside the corresponding analtyical result.

expressed as %
Penalty from
(time, fuel)
0.11%0.8%, 1%
0.24.2%4.2%, 4.2%
0.39.9%10%, 9.9%
0.4 19%19.2%, 18.8%
0.5 33%33.3%, 32.9%
0.707100%99.2%, 97.8%

Clearly there is very good agreement between the simple analytical result and the more detailed calculations from  iNavCalc, confirming that when the wind blows, we lose out on the round-trip.

But what about other scenarios that are not directly into-/down- wind ? We could quite easily derive analytical expressions for these scenarios, but they would not be as simple because of the need to include angular offsets and trigonometry. Instead, we can simply use  iNavCalc to explore these scenarios.

Let's start with the intermediate crosswind situation where the wind is off the nose by 45 degrees on the outbound leg (and likewise 45 degrees off the tail inbound). This is simple to evaluate using  iNavCalc: all we have to do is re-specify the wind direction. For example, the  iNavCalc parameter string for a wind of 045 degrees at 10 kts would be:

route={540500N 0043724W MyStart}, >360/100, >180/100;  ias=96; alt=3000; met=manual; oat=15; qnh=1013.25; wind=045/10

The table below shows the results computed by  iNavCalc for the range of windspeeds at 045 degrees, compared with the corresponding penalty for the into-/down-wind scenario.

for into-/down-wind
Penalty from
results for wind
at 045 degrees
(time, fuel)
0.11%0.8%, 0.7%
0.24.2%3.3%, 3.1%
0.39.9%7.5%, 7.4%
0.4 19%14.2%, 14%
0.5 33%25%, 24.4%
0.707100%73.3%, 71.8%

The penalties are still inevitable, though are not as severe as for the direct into-/down-wind scenario.  Finally, let's explore the pure crosswind scenario, wherebey the easterly wind is specified in the  iNavCalc parameter-string as:

route={540500N 0043724W MyStart}, >360/100, >180/100;  ias=96; alt=3000; met=manual; oat=15; qnh=1013.25; wind=090/10

for into-/down-wind
Penalty from
results for wind
at 090 degrees
(time, fuel)
0.11%0.8%, 0.5%
0.24.2%2.5%, 2.1%
0.39.9%5%, 4.8%
0.4 19%9.2%, 9%
0.5 33%15.8%, 15.3%
0.707100%41.7%, 40.8%

Again, the penalties are inevitable (some fuel must be burned to counteract the crosswind), but are the most benign compared with the direct into-.down-wind scenario.

Gathering all these results together, the following conclusions can be drawn:

  • Whenever the wind blows, there will inevitably be a round-trip cost in terms of time and fuel compared with the zero-wind scenario
  • The worst-case penalty occurs when the wind is a direct headwind/tailwind. In which case, the round-trip percentage penalty is approximated by the analytical expression 100*r2/(1-r2) where r=W/V
  • The least penalty occurs when the wind is directly across the track. In which case, the round-trip percentage penalty is approximately half the worst-case, i.e.,  50*r2/(1-r2where r=W/V
  • The intermediate case when the wind is 45 degrees off-track gives a round-trip percentage penalty of approximately three-quarters the worst-case, i.e.,  75*r2/(1-r2where r=W/V. This is a useful rule-of-thumb when initially planning a trip.
  •  iNavCalc is a powerful and convenient tool for performing such calculations for training/educational purposes (as in this article), or for actual flight planning using its built-in automated weather feeds. Sign up now -- it is *free*.

Remember -- it is wholly incorrect that the tailwind on the way back will 'blow you all the way home' and make up for the headwind on the way out. In fact, this is the worst-case scenario in terms of time and fuel penalty on a roundtrip. Pilots beware!

Saturday 28 May 2011

Pilots...don't get caught out of the cloud !

The cloud is the hottest thing in computing...and it makes perfect sense: outsource the messy business of hardware and network infrastructure (which is a specialization most of us are not very good at), so we can focus on developing and consuming software-based services. The recent explosion in cloud services has come about with the convergence of virtualization technology (whereby many different servers at the software level can cleverly share the same hardware without getting tangled up) and  multi-core processors (where multiple CPU cores -- which do the heavy-lifting inside a computer -- can share the same physical ancillary services such as memory, network connectivity, storage, power supplies, cooling fans, rack-space, etc).  With the result, we now have, in essence, the commoditization of computing infrastructure: it no longer really matters where the servers sit.  From our perspective as developers and consumers of services, the computers are somewhere out there in the clouds, accessible via the URL (the detail is of course a bit more complex, but the concept is essentially that simple).

For pilots, like anyone else, we can expect the cloud to have an ever-increasing impact on our lives in terms of the rapid expansion of cloud-based services. For example, an obvious and natural "fit" for a cloud-based pilot app is the virtual (web-based) logbook. As such there is a proliferation of offerings in this area.

As a "pet project" (during the long winter months here in the UK), I decided to embrace the cloud by creating iNavCalc (and iMetBrief), web- and email-based apps for VFR pre-flight planning. On the principles that (i) humans shouldn't do what computers can do better; and (ii) you should never do twice what you can teach a computer to do once, I have outsourced to the cloud the considerable and repetitive grunt-work of pre-flight VFR nav planning. I find that these apps save me hours compared with the old way of doing things. I use them every time I fly. It now takes me seconds or minutes rather than hours to pre-plan a flight. Because I find them so handy, I thought other pilots would, too. So, I've recently launched them as a free service for the global flying community. The apps are platform-independent: all you require is a smartphone/tablet/desktop that supports email, web-browsing, and can display PDF files. I'll talk more about the details in later postings. For now, give them a go (they are free)... and do please let me know what you think.

Fly Meister

Sunday 22 May 2011

Fly the famous Isle of Man TT Mountain Course

The world-renowned TT motorcycle road-racing season is kicking-off on the Isle of Man. Over the next few weeks (30th May to 12th June 2011), this erstwhile tranquil & sleepy island in the middle of the Irish Sea transforms itself into petrol-head heaven. Mostly of the two-wheeled variety, but no need to get left out if you prefer two wings to two wheels. You can fly the TT course (above 3000 feet, you need to comply with air traffic at EGNS -- they operate TT airspace restrictions during the races, details by NOTAM). In a typical light aircraft you'll have little chance of beating the times achieved by the bikes. They complete a lap of the circuit in 15 minutes, reaching 200 mph or so on the straights. I've tried it in my Bulldog. Best I could achieve was little over 20 minutes... Click here to open the TT route in iNavCalc (requires registration).

Friday 20 May 2011

Announcing iNavCalc and iMetBrief

Update: iNavCalc V3.0 mobile app now released

Update: all described functionality now available via mobile app...

Flying season is upon us, so we'll cut to the chase: check out our brand new, free-to-use, flying apps designed by pilots, for pilots. First up is iNavCalc, our mega-powerful feature-packed email and web-based route navigation planning tool, complete with AutoMETic wind and weather calculations. To get started, send a blank email to and check the email response for further instructions. Check the documentation for detailed instructions.

Next up is iMetBrief, our simple but massively convenient TAF and METAR app which basically does what it says on the tin. Send a blank email to to get started.

(Our apps are freshly-minted, so don't expect perfection: simply use them if you find them useful, don't if you don't. Follow us on Twitter to keep in touch.)

Toes clear...
Fly Meister

Friday 1 April 2011

AutoMETic for the people

If you're a GA pilot, like us, you are likely frustrated at the plethora of tools and resources out there -- some free, others paid -- to help you with your flight planning. Given all the available resources, you might reasonably ask "why can I not log on somewhere, put in my route, and get the weather forecast and PLOG ?"  The answer to that simple question, as far as we can tell, is "because such a site doesn't exist" (at least not for FREE). That in a nutshell has led to the development of this site: namely, to provide a web-based resource where pilots can access FREE tools to provide the key information they need to conduct their flight. Moreover, to be of any real use, the tools must work equally well from the desktop, tablet, or smart-phone. That is the challenge we have set ourselves.

Our aim is to provide a suite of tools to address these and other GA-oriented goals. All these tools are built "by pilots, for pilots" so you can be sure we have considered convenience and usability. If the tool isn't easy to use in the real world, we will not post it up.

Your patience is appreciated since (i) please remember these resources are being published FREE of charge to you; and (ii) we are obsessive about the need to make the tools usable not just functional. That means continuous tweaking, re-testing, re-tweaking until we get it it right.

First up will be a MET and PLOG web application (currently in final stages of testing) which will enable you to get current MET and generate PLOGs for any route in the world.

The application is aimed at GA pilots on VFR flights. Main features:
  • Ability to import a route from the open-standard GPX format ( Various third-party software tools support this format, such as Memory-Map ( For example, we are currently testing our app using Memory-Map software running on an Ipad: we enter the desired route (as a "route") in Memory-Map, and send it by email to FlyLogical (via the export-to-email button in Memory-Map). Our app then sends back (as email attachments) the current MET and PLOG reports for the route.
  • The PLOG computation for drift uses the current MET (from METARS, TAFS, and global weather forecasting software) to determine winds aloft for each waypoint along the route (we call this "AutoMETic")
  • The PLOG computations for magnetic heading use a numerical geomagnetic model to calculate the magnetic deviation for each and every waypoint for the current calendar date (i.e., it doesn't just perform some kind of lookup on previously-computed data grid which can grow stale when the magnetic field of the earth shifts as the years go by).
  • The PLOG calculations provide altimetry (transition levels, flight levels, etc) and performance  (density altitudes, freezing altitude, etc) calculations along the route (again using AutoMETic to determine localised temperatures and pressures for the basis of the computations).
  • The PLOG calculations provide solar angle calculations (azimuth and zenith) along the route corresponding to the dates and times along the route, enabling, for example, to determine if any of the flight is conducted in darkness, or if any of the flight will be conducted "into the sun" (possibly affecting visibility) etc.
  • The PLOG calculations include fuel consumption profile along the route (including consideration of fuel consumption for initial climb-out, and any en route changes in altitude) 
  • The PLOG calculations include range and endurance estimation for possible diversion from any point along the route, based on remaining fuel along the route.
  • Ability to specify wide range of parameters for the PLOG calculations including altitude profile along the route, airspeed profile along the route, date and time of departure (including UTC offset for local time), waypoint at which the stopwatch is zeroed (e.g., to enable PLOG times to be computed relative to a specified starting waypoint which can be different from the departure airfield).
  • Ability to save routes and parameters for convenience and later re-use (still FREE, but requires registration)
  • Ability to set and save preferences such as aircraft performance parameters (IAS, climb rate, fuel consumption, fuel capacity, etc) used in the PLOG calculations (still FREE, but requires registration)
  • Ability to set and save preferences for dimensional units (feet or metres for altitude, mph or kts for speed, mb or InchesHg for pressure, etc)  used in the PLOG parameters and in the generated output report (still FREE, but requires registration)
We are excited about the finalisation of this feature-rich FREE service which we will be publishing on this site imminently. 

Toes clear.