Hello all! I’ve been flying a cross America trip in a DA62 and I’ve made it up to Montana. As I got into the mountains temps dropped and I found myself flying in -5C to -19C weather. My first leg this was fine, the second leg had me going through a pretty big storm cell. I got lots of rain and lots of ice. I had all de-icing on but at some point my airspeed indicator froze and was stuck and only freed minutes before landing. I still had the tiny KT speed below but knew it was a different unit and was unfamiliar how it compared to the speed units for my normal display (Which I’m assuming is nautical miles per hour?)
Is there any system to prevent the airspeed sensor from icing? Or is it just something you have to be aware of and be ready for?
Look for the large white pushbutton below the PFD that says “pitot heat”. That should normally be turned on when you’re flying. This is separate from your aircraft anti-icing/de-icing equipment. Doing this will also turn off the alert on your PFD that says “Pitot heat off”.
If you’re unsure what an aircraft pitot is, you might want to goggle it for a better explanation than I could give in a short discussion here.
Regards
Fun fact: that‘s how it was translated into german in the original release version of the sim. Pilotenrohr… pilot tube. Whatever logic tells what that should be
The pitot tube is a tube usually attached to the wing or the nose of an aircraft that has a connection to the airspeed indicator. It measures the ram air, means the more the streaming air presses into the tube the more it pushes against a surface inside the airspeed indicator and the more speed the needle shows. If this pitot tube builds ice less air can press into it and the less air presses against the surface in the ASI. Pitot heat is an electrical heating of this tube to prevent and remove ice.
Normally you don‘t use pitot heat all the time but only in temps that allow for freezing conditions as hot air has another density than cold air and the indicated airspeed shows incorrectly (slower). The yellow notification is just that: a notification.
The other half of that system is the “Static Port”. Speed is measure as the difference between the static air pressure and the ram air pressure. While there is a way to deice the Pitot Tube, there is no provision for deicing the Static Port. This is not normally a problem as the Static Port is normally mounted on the side of the fuselage and is not normally affected by ice. However, in cases of freezing rain the Static Port can become obstructed.
The Static Port provides ambient air pressure to the Airspeed Indicator, Altimeter and Vertical Speed Indicator. There is an Alternate Static Source available, usually as a small plunger type control that will allow for switching to a static source inside the cockpit on unpressurized aircraft. If not equipped, in an emergency, breaking the glass on any of the static port instruments will restore a static reading, albeit slightly lower than the primary port.
Quiz time.
What happens if the Static Port gets obstructed while cruising, but the Pitot Heat prevents or removes any ice obstructing the Pitot Tube?
Answer - Don't cheat. Post your answer first, then read this.
Pitot-Static System Failure:
Blockages in the system can cause a variety of errors
To prevent these errors you must complete a thorough pre-flight
Blockages can occur from FOD, striking an object, insects, and icing
At the altitude where the Pitot tube becomes blocked, the airspeed indicator remains at the existing airspeed and doesn’t reflect actual changes in speed
At altitudes above where the Pitot tube became blocked, the ASI displays a higher-than-actual airspeed increasing steadily as altitude increases
At lower altitudes, the ASI displays a lower-than-actual airspeed decreasing steadily as altitude decreases
Pitot-Tube Drain Hole Blockage:
Would cause ASI to malfunction
Pitot-Tube and Drain Hole Blocked:
Would cause ASI to malfunction
Static Port Blockage:
Would cause ASI, altimeter, and VSI to malfunction
If in a real emergency, malfunctions can be corrected with alternate air or breaking the glass on a Pitot static instrument (VSI)
If the static ports become blocked, the ASI would still function but could produce inaccurate indications
At the altitude where the blockage occurs, airspeed indications would be normal
At altitudes above which the static ports became blocked, the ASI displays a lower-than-actual airspeed continually decreasing as altitude is increased
At lower altitudes, the ASI displays a higher-than-actual airspeed increasing steadily as altitude decreases
The trapped pressure in the static system causes the altimeter to remain at the altitude where the blockage occurred
The VSI remains at zero
On some aircraft, an alternate static air source valve is used for emergencies
If the alternate source is vented inside the airplane, where static pressure is usually lower than outside static pressure, selection of the alternate source may result in the following erroneous instrument indications:
Altimeter reads higher than normal
Indicated airspeed (IAS) reads greater than normal
If you have pitot heat installed you should ALWAYS use it as the magnetic compass is calibrated with pitot heat on. The pitot heat does not have a significant effect on airspeed indication, I have never seen a correction table for pitot heat ON versus OFF.
In short, if the pitot becomes blocked the airspeed indicator will act like an altimeter.
If a single static source becomes blocked, the pressure instruments readings will become erratic and unreliable in a slip or turbulence.
If both static sources becomes blocked the altimeter will show whatever it was showing before the blockage occurred, vertical speed indicator will show zero and airspeed will over read during descent and under read during climb.
The quickest way to determine if you have an instrument failure or a static source blockage is to check the vertical speed indicator, if it still works, its an instrument error. If not its a static source problem and airspeed is unreliable.
Be aware that on more advanced planes the vertical speed might be coming from an inertial unit instead of pressure driven .
I haven’t either but I’ve been tought to only use it in visible moisture or below 5°C. Our manuals don’t say anything about it specifically but the Cessna POH mentions to turn it on when inadvertantly flying into icing conditions, the checklists say “as required”.
I could imagine that they teach it so to manage it purposely and don’t forget to turn it off. You can really burn your fingers and I guess it’s not healthy for the probe to overheat on the ground with no cooling airstream.
Thats why you should turn it ON during line-up and OFF when vacating the runway, I have little experience with Cessna’s any other GA aircraft I have flown worked that way. I have seen the Pitot heat cause more than 20 degree heading changes on the magnetic compass on certain aircraft. The magnetic compass is compensated for the magnetic field of the aircraft, usually with pitot heat ON.
All transport category aircraft have probes heated all the time, usually on air-ground logic. The Embraer automatically heats probes when one engine is running.
I’m currently googling a bit, PPRune etc… It seems it’s standard in commercial aircraft to always have it on. I’ll check the C172 POH and try it out next time… however I can’t remember to have noticed anything in the past. Interesting topic, I didn’t even think of the compass.
What aircraft are we talking about specifically? I’m speaking in general, all GA aircraft I have flown have compass swing performed with pitot heat ON. You should maybe ask your maintenance organization what they are doing, if they perform compass swing with pitot heat OFF then I assume thats the normal configuration and you should keep it OFF. I have never seen it that way though.
The poster was talking about the DA62. I assumed that’s the aircraft being covered. As for the compass, I’ve never made that check. I’m give it a go to see what happens.
“DA62 is a twin engine touring motor glider…”
That may be, but it’s also not a transport category aircraft.
The maintenance manual should state in what configuration compass swing should be performed, based on that it needs to be either ON or OFF to give an accurate reading.
Same for windshield heat, if installed it should usually be turned OFF when using magnetic compass as it creates a very strong magnetic field.
I flew one aircraft which magnetic compass was only useable with both windshield heaters and both generators OFF.
Quite smart in a way, I’m not using the magnetic compass anyway unless really hits the fan, which means losing both electrical systems, which automatically means losing both windshield heaters and both AHRS .
As for the DA62 and all other aircraft with the fancy screens, they all have flux valves somewhere in a wingtip usually, far away from any disturbances caused by the aircraft electronics. So maybe not that important?
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really hits the fan, which means losing both electrical systems, which automatically means losing both windshield heaters and both AHRS