Heated Pitot.

[Pushboy]

Hi— I’m starting the process of planing the electronics in my kit and I’m looking for some real world advice on the Garmin heated pitot options. I’m wondering if the regulated version is of any value in a Kitfox or is the simplicity of the unregulated version better?
Any advice/experience would be valuable.

Thanks— Dave


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[Geek]

Hi— I’m starting the process of planing the electronics in my kit and I’m looking for some real world advice on the Garmin heated pitot options. I’m wondering if the regulated version is of any value in a Kitfox or is the simplicity of the unregulated version better?
Any advice/experience would be valuable.

Thanks— Dave

Dave - I went with the -20 regulated heated version and think if I was to do it again (meaning if I had read the book closer) I would probably have gone with the -10. I never plan on flying my plane in cold weather where I would need it for icing (granted you're a bit colder there in Ottawa where a friend of mine there skates to and from work on the river in Winter) but I do use it/want it when flying in rain. In Table 2-1 on page 2-16 (rev AT manual) it talks to current draw between the two heated probes. What you might be interested in is contained in the notes and I put the part you might be most interested in in bold. Here's a mock up of the table.

"Table 2-1 GAP 26 Initial Current Draw vs Probe Temperature (-10, -20 units only)
Probe Temperature -40°C (-40°F) - 12A, 0°C (32°F) - 9.25A, 50°C (122°F) - 7.3A, 100°C (212°F) - 5.85A, 175°C (347°F) -4.36ANotes (from the table in the manual)
Table 2-1 refers to probe temperature at initial turn-on. For example, upon turn-on at -40°C, the standard
(-10) or regulated (-20) heated probes will initially draw 12 amps, but will draw less current as they warm
up.
The -10 non-regulated probe draws current proportional to the probe temperature as shown in Table 2-1.
The -20 heated regulated probe uses similar power to the -10 probe when airborne, but will limit the
current to regulate the probe temperature to ~75°C (167°F) when the probe temperature reaches this
value."

Hope this helps.

Gary

[Eric Page]

This isn't directly related to your question, but it's something that many builders don't think about when adding electrical loads to their airplane.

If you're using a Rotax "iS" engine, be sure that you have a step in your checklist to shed the pitot heat in the event of a stator/regulator failure. Pitot heat will likely be the highest non-engine electrical load in the aircraft and will greatly accelerate battery discharge. In the unlikely event of a dual stator/regulator failure, the engine will only run as long as the battery can sustain it, so electrical load shedding is critically important. This applies to any other electrically dependent engine, not just Rotaxes.

Assuming a lithium battery and taking EarthX as an example, the discharge curve for their ETX-900 (15.6Ah; just a tick under the Rotax-specified 16Ah) shows that by reducing load from 12.4A to 6.2A (pretty close to turning off pitot heat), time-to-discharge is doubled.

ETX-900 Discharge Time.jpg

As you can see from the curves in that chart, when lithium-based batteries get near fully discharged their voltage suddenly falls off a cliff (and the higher the load, the steeper the cliff), so accurately predicting battery-only engine endurance based on measured battery voltage is very difficult. When you notice voltage falling you may have seconds until engine failure. Battery Management Systems add a big unknown to the equation, as most battery manufacturers don't publish the exact voltage at which their BMS disconnects the battery to prevent over-discharge.

For example, the EarthX battery manual says this [page 2, "ETX Series - BMS"]:

The BMS disconnects the battery from the load if 100% of the usable energy is consumed. The usable energy is the rated Ah of the battery (new battery at 25DegC, see the Specification section within). [...] An over-discharged battery typically has an internal voltage less than 11.0V, but when the BMS disconnects, the voltage reading at the terminals of the battery will be zero volts.

Then later, it says this [page 40, "If your battery is at zero volts."]:

EarthX lithium batteries have over discharge protection via an internal Battery Management System (BMS) to disconnect the battery from the active load (your vehicle) to protect the cells from damage when it is 95-98% drained.

The manual only hints at the exact voltage where the BMS disconnects the battery from aircraft loads, so watching the voltmeter to predict cutoff is a fool's errand. Bottom line: with an electrically dependent engine and a lithium battery, when the charging system fails it's time to start aggressively saving electrons.