SEN 2880 - Operation of the All-Tee altimeter - Pathway- AMA Nats FF intro

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SEN 2880

1. Operation of the All-Tee altimeter
2. The Pathway
3. AMA Nats FF intro

Operation of the All-Tee altimeter
From: Allard van Wallene

Operation of the All-Tee altimeter



Most if not all altimeters used for aeromodeling are based on measuring the air pressure the sensor is exposed to. This pressure drops when the height of the model increases. A processor in the altimeter converts this pressure to the height of the model. I am using ‘height’ instead of ‘altitude’, because altitude is typically used to express the elevation relative to the sea level. However, our model’s elevation is relative to ground level so ‘height’ is the better expression for it. If you want to play around a bit with pressures and altitudes, this web page gives you the possibility to enter a pressure and the altitude is shown: <>
So as said, the pressure is converted to the height of the model in meters relative to the ground level the model was launched from. The altimeter electronics automatically ‘zeroes’ the ground reference when it is being switched on, data are being read from its memory and when the memory is cleared (flushed). For an equal playing ground between competitors the flushing of the memory is best performed with the model on the ground, as this would be the ideal zero-reference.

For those who want to read up on the math behind the pressure to altitude  conversion, here is one of the many web pages, which explains it: <>
Of course there are many factors that influence the accuracy of the pressure to height conversion, ambient temperature for one. But since these are the same for all those using the altimeter in a fly off, the deviations in height are also the same for each competitor. All we are interested in, is the difference in height between the models to determine the winner. If an altimeter is ‘off’ by plus-one meter, it is so for every competitor.

The competition, height and time:

A sporting code rule reads that if a competition cannot be concluded due to poor visibility or terrain conditions (size), an alternative fly off may be flown using the height of the model at a predefined flying time of no less than 2 minutes. So now the time comes into play as well. The time base inside the altimeter is quartz controlled with a maximum error of 10 ppm (that’s parts per million). For a flight of 10 minutes this boils down to 0,006 or 6 thousandth of a second. That is two orders of magnitude more accurate than ‘rounding off to the second’ territory, so nothing to worry about.
More critical is determining the moment of launch. When does the flight time start? All-Tee does not determine the start of the flight automatically. For models, which are launched from the hand (e.g. F1B and F1C) this could be a simple algorithm. For F1A it is a whole different ballgame. The model can be released from the towline anywhere between 53 (50 meters plus the length of a big guy with his hand stretched above his head) and 1 meter (where the model floats off the towline close to the ground). There is no syntax, which can effectively determine this. So the moment of ‘launch’ has to be set manually by setting the time = 0 there. This can be relatively easily seen in the height-graph downloaded from the model, as it will show a discontinuity at exactly the time where the model jumps away from the towline or is being launched from the hand. The graph can be zoomed-in to tenths of a second level and the time can be set to zero (start of free flight). There is of course some room for error, however this is within a few tenths of a second. If the setting of the moment of launch is e.g. off by 0.2 seconds, at a rather high flight sink rate of 0.5 meters per second (bad air) this boils down to a height error of 0.1 meters (10 cm): the same order of magnitude as the resolution of the altimeter. In other words, the error induced by getting the moment of launch not exactly right will not make or break an eventual winner, where the height used is typically rounded to whole meters anyway.

 Then there was some discussion as to when an altimeter can be actually used in competition. Currently there are two different rules in the FAI Sporting Code (I invite the SEN readers to find and read them on the FAI web pages before entering the discussions). It can be used in regular fly offs where the time recorded by the timekeeper is, according to the sportsman, wrong. This can have many reasons and we all know the situations why this can happen. The sportsman can challenge the timekeeper’s recorded time by showing the recorded altimeter graph to the contest director or jury. The actual flight time from launch to landing can be accurately determined.

In the past, so called DT-fly offs were used, but this was outlawed by the FAI. As a fair alternative to such fly offs, another sporting code rule relates to so called ‘alternative fly offs’ where field and/or visibility conditions do not allow a normal maximum flight time. In this case the contest director may decide to use this rule, where the height of the model at a preset flight time (but no less than 2 minutes) determines the winner. There will always be sportsmen who do not agree with such a fly off, but the rule is there, accepted by a majority vote within the FF technical subcommittee and now part of the sporting code. To put it bluntly, if you have no interest in altimeter fly offs you can either decide not to fly or participate in the fly off and get a zero height score. Of course if nobody makes the preset flight time, the longest flown time counts. As an example, a score could look like this:


1. Participant A    240     240     180     180     180     180     180     240 +56m

2. Participant B     240     240     180     180     180     180     180     240 +51m

3. Participant C     240     240     180     180     180     180     180     240 +32m

4. Participant D     240     240     180     180     180     180     180     240 +10m

5/6. Participant E   240     240     180     180     180     180     180     240 +0m

5/6. Participant F   240     240     180     180     180     180     180     240 +0m

7. Participant G     240     240     180     180     180     180     180     232

8. Participant H     240     240     180     180     180     180     180     119


Here, the contest director decided to organize an alternative fly-off using altimeters and a max flight time of 4 minutes, as the wind had increased such that a regular 6 minute fly off would carry the models into a nearby village. Participants A to D used an EDIC certified altimeter and made the max time of 4 minutes. The altimeter showed that the flying heights at 4 minutes were 56, 51, 32 and 10 meters respectively. The highest model (56 meters) won.

Participant E made the max as well but did not have an altimeter, therefore no height could be established resulting in a zero height score. Participant F did have an altimeter, but a power supply malfunction resulted in no recorded height and therefore also a zero height score.

Participants G and H used altimeters but did not make the max. Altimeter score was therefore zero.


Of course, certified altimeters must be freely available otherwise a sportsman can never be penalized by a rule which cannot be used because altimeters are not available. However, there are still some 50 altimeters in stock, so this argument does not (yet) hold ground. If a participant decides not to use an altimeter when they are readily available, it would be similar to a participant who decides not to buy rubber, towlines, mandatory RDT for F1C or other flight essentials, which are defined in the sporting code. If you want to play a golf match you must have a set of clubs. If you want to fly an alternative fly off you must have an altimeter if you want to have more than a 0 height score.

The future will show how such alternative fly offs play out. The CIAM FF technical subcommittee will follow all the developments closely and adapt (=clarify) the current rules if deemed necessary.

The Pathway

With increasing performance and decreasing field size FAI Ff flyers were looking for ways to bring an event to a conclusion with a proper sporting conclusion. There was a suggestion to do what because known as the altimeter flyoff using altimeters that many participants already had in their models. There were also some inexpensive ones readily available. This gave rise to the famous “altimeter dance “ to put a unique signature on the altimeter record.

While this approach was by no means perfect it was very important positive decision to do this as it indicated that the idea was feasible but a few improvements were needed. It was important to at least try something otherwise we could have gone around in circles not doing anything. And a similar experience on using an altimeter to dispute a flyoff miss timing.

From this experience together with “encouragement” from the CIAM and information from the EDIC who already had experience in certifying altimeter devices for R/C soaring events the later version of the sporting code was written. The first device conforming to these standards is the All-Tee.

We see from the comments that some further tweaks in the procedures may be required but we are moving the right direction.

As a side comment this has been done in a climate of continual improvement of the pressure sensing devices used to determine the altitude. This has given increasing accuracy and diminishing size and cost. For example, one of the suppliers of these sensors is the well known company BOSCH who over the last 10-15 years have come out with 4 major evolutions of the kind of pressure sensor we could use.

Where will the next steps go …

The target for many is to be able to automatically time the flights. Here one of the major technical challenges is how to determine accurately the end of the flight. No doubt people are thinking of that.
There was a very interesting post on FB recently by the well known F1A flyer Per Findahl who said what about using this technology to post real time flight information, even during the flight on the internet. This is not as far away as it might sound. In some events these , for example the last World Champs the score board was visible on the internet in real time as the scores were recorded and many flyers on the field used their smart phones to check status instead of going back to look at the score board and people off the site and in other countries checked it regularly. The radio systems that are used by many RDT systems are off the shelf 2-way radio could easily report back to the contest director when the flight ends and to the press office and internet on flight progress including height. While this may seem extreme and perhaps tech exhibitionism to some, things like this promote interest and attract the young.

Finally Free Flight has many classes to suit interest, budget, physical abilities etc. So this is not for everyone. You would not use it for a catapult glider event for 20 school kids… or would you if it was a STEM class ? The All-Tee and devices like it do cost money but less than many would pay just to travel to one event.


AMA Nats FF intro
The US Nats or National Championships runs over several weeks with different disciplines each week. This year the AMA did an into YouTube video on each discipline. The one on Free Flight was narrated by the Outdoor Free Flight Contest Director Ed Carroll. Ed is a long timer SCAT member, power flier and F1C regular. You can see it here:

The AMA regularly does video and podcasts and info is on their web site The Free Flight Nats is run by the Special Interest Group NFFS and their web site is