Understanding F5J Start Height

Page Last Updated: 01-13-2015

For beginning F5J pilots there is often initial confusion regarding the F5J Start Height parameter. So first let's look to the FAI regulations for F5J, then we'll walk through several examples.

From the FAI regulations for F5J, section h) (bold emphasis added):

The essential functions of the AMRT (altimeter/motor run timer) are:

i) To record and display the maximum height attained (Start Height), above a ground level reference between the instant of motor start and 10 seconds after the motor is stopped and

ii) To restrict the operation of the motor by the competitor to a single continuous run not exceeding 30 seconds.

For convenience let's define the "launch phase" as the time between motor start and 10 seconds after motor cutoff. So the F5J Start Height is the peak height flown during the launch phase. A common misconception is that Start Height is the height measured at 10 seconds after motor cutoff. It's not.

From a scoring perspective here's an extract from FAI regulations e):

Each meter of the recorded Start Height results in a deduction of half (0.5) a point up to 200m and three (3) points above it.

In other words the more your Start Height exceeds 200mtr the more you will be hit with that costly 3 points per meter penalty. Example: say that during the launch phase your plane hit a maximum of 215mtr. Your Start Height penalty would therefore be (200 * 0.5) + (15*3) = 145 points. On the other hand if your plane only hit 180mtr during the launch phase then your Start Height penalty would be (180*0.5) = 90 points.

You might wonder if you could legally use telemetry to send back the current altitude of your plane and keep you from crossing that 200mtr line. Answer: nice try but no! (FAI d)Any device for the transmission of information from the model aircraft to the competitor is prohibited.)

Let's look at several F5J launch examples.

F5J altitude-example 1

In Example 1 (figure above) the plane does a fairly steep climb for ~17 seconds and then keeps the motor on for a remaining less aggressive climb until the motor cuts at the 30 second maximum. From there the "zoom" climb continues for about 6 seconds and then the pilot dives his plane 30 meters before the "motor off + 10 second" marker. The F5J Start Height of 160mtr is the highest point between motor on and motor off + 10 seconds. i.e. the dive after the zoom did absolutely nothing to reduce the Start Height penalty.

F5J altitude-example 2

In Example 2 (figure above) the plane executes a steady steep climb up to 220mtr and then dives until the pilot cuts the motor at 200mtr and ~26 seconds. The plane continues to dive until it reaches an altitude of ~160mtr when the pilot starts a zoom climb. Note that in this example the F5J Start Height of 220mtr happened before the motor was cut i.e. it is the peak altitude between motor on and motor off + 10 seconds.

F5J altitude-example 3

The launch in Example 3 (figure above) is a typical aggressive fly-off launch where the pilot is trying to hook into a low thermal and minimize his Start Height penalty. So in this example the motor is cut by the pilot at 10 seconds and 25mtr height (note the plane's launch speed was likely slow to allow the pilot to see low thermal bumps better). So once the motor was cut the pilot immediately circled in a low thermal and started climbing immediately from motor off to beyond the motor off + 10sec marker. Therefore in this specific example the F5J Start Height is also the height measured at the motor off + 10 seconds mark.