Carby Ice? I Think Yes, ATSB Don't

[poteroo]

See ATSB: Report AO-2017-046

 

Relevant as we are coming up to winter and more humid conditions in southern states.

 

Comments?

 

 

[Methusala]

“It’s when things are going just right that you’d better be suspicious. There you are, fat as can be. The whole world is yours and you’re the answer to the Wright brothers’ prayers. You say to yourself, nothing can go wrong… all my trespasses are forgiven. Best you not believe it.”

 

— Ernest K. Gann, advice from the “old pelican,” The Black Watch, 1989.

 

 

[danny_galaga]

See ATSB: Report AO-2017-046Relevant as we are coming up to winter and more humid conditions in southern states.

 

Comments?

Isnt winter generally less humid?

 

 

[pmccarthy]

If engine was warmed up then for a PA 28 at full power icing is most unlikely. For a PA28 Carb Heat is not recommended routinely in landing circuit unless indicated by conditions, which is different from C172 for example, because intake air is heated by the engine to some extent anyway due to the position of the intake duct.

 

 

[Birdseye]

Based on my PA28 experience in the UK (similar conditions, if not a touch colder), I'd agree with the report and say very unlikely that carb ice would occur under full power take off conditions. I certainly encountered it at cruise settings.

 

 

[Methusala]

On my Vision, I have installed an electric carburetor heater. the instructions say to use heat on take off.  The fact that the electric heater does not heat the incoming air but instead the carb throat, the prophylactic use of heat at takeoff shouldn't degrade power and so imo, given that takeoff is the most power critical stage of flight, use of heat is a good idea.

 

The accident report is (relatively) open on the possibility of ice as the cause of power loss but it then leaves a mystery as to what other factors may be in play. Perhaps use of heat, in my case, is a judicious course to follow.

 

Don

 

 

[Thruster88]

Carb heat is never used for take off, if carb ice had formed the rpm  in the initial run would be low and the take off should be rejected.

 

I believe that changing tanks just before take off is a big mistake, select the tank before engine start. If there is a problem with a tank

 

Blockage, water etc it  has more time to become apparent. 

 

 

[Thruster88]

This one AO 2016-082 is very interesting, a case of forth time unlucky. The chief engineers comments are worth a read.

 

 

[Fred]

Without referring to this specific incident, the run up is ideally done close to the holding point and without icing you should observe a drop in RPM with carby heat applied -  if you get an increase you are making ice.

 

 

[Camel]

I'm a former resident of Jindabyne in the Snowy Mountains ! I started my C172 taxi for a bit and warmed up, did checks ! Taxi to other end and lined up ! Quick check and rolled ! On climb the engine spluttered badly ! Heat applied and with a paddock lined up on descent the engine came good and climbed !  Never again will I do run ups and checks until I'm close to take off point or at least run with heat on after taxiing ! 

 

 

[kgwilson]

Isnt winter generally less humid?

I agree. Winter is generally far less humid than other seasons in temperate climates. In other latitudes I am not so sure but Winter in the Tropics is also the dry season so presumable less humid as well.

 

 

[facthunter]

 You do need moisture for carb icing and there's not much moisture at the south pole The highest absolute humidities occur at the hot maritime locations or hot localities after rain .

 

        The causes of carb icing are a combination of a few  factors,  Ambient air temp, humidity and engine characteristics including engine installation. Long manifolds (unheated )on engines fitted  with carburetter(s)  are the worst.  Lycoming engines often have the inlet routed through the sump so get quite warm in the intake.You can get another type of ice on forward facing intakes with filters ( rhyme) that will also form on wing leading edges ) which can rob the engine(e) of power by making them run  rich, as well as reducing mass airflow. The problem with icing is it's lack of absolute predictability, so you must play safe. IF you think it's a" possibility "use it .  Prevent it rather than have to correct it.

 

      Hot air for many engines is not filtered so you can get a lot of dust into some engines if you use the hot air when taxying.. Dust can accumulate in the Muff so even if there's no dust about you might be putting abrasive stuff into the engine  from what's there. Perhaps modify that design, where it exists, if you can..

 

  With Carb heat, use ON or OFF. One or the other and if applied give it time to act unless the power drop will put you in peril. . A little bit of heat not applied for long can worsen the situation. If your engine isn't delivering much power, as with an  idle descent. The heat will not be in the exhaust system tor you to effectively de ice the Carb.  Apply carb heat before reducing power, and warm your engine occasionally on long descent or plan a "powered" descent.

 

   The evaporation of the fuel in the carburetter throat and the venturi effect cause a lot of heat to be used so that can cause the icing at quite high temperatures even well above 20 degrees C  A high humidity situation makes the  amount of icing and  a higher rate of formation more likely.  Low cloud base gives a good indication of "relative" humidity

 

  Carb heat left on will significantly reduce power  so get in the habit of" power UP, check carb heat OFF" in the same motion. ALL the time.   Nev

 

 

[Yenn]

In Qld it is far more likely to get carby ice in summer because the relative humidity is high. I would never expect ice on take off, but I had to abort a C150 take off at Grovedale in Vic in mid winter.

 

Some people say you don't get icing in two strokes, but I have had it in Qld summer humid conditions.

 

 

[poteroo]

My Points of Difference with the ATSB Report AO-2017-046

 

1. ICING PROBABILITY

 

Chart shows only moderate icing probability at cruise power - but serious at descent power. -   Perhaps more than serious at idle power on the ground?

 

The engine was cold when started at 1109. The carby heat was checked and found operable during engine runup. -  what isn't known is whether the pilot ensured that there was no ice formed already in the carby by selecting it on HOT for at least 20 secs. This was also not carried out immediately prior to entering the runway after receiving clearance. In suspect icing conditions, ensuring that the engine is free of ice before takeoff is, (IMHO), an absolute essential, and is what we teach. In other words, engine run-up last, and carby heat left on for 20 secs to ensure any ice already formed is melted.

 

The aircraft commenced takeoff at 1114, soon after the aircraft was airborne and past the threshold, the pilot noted power loss. Then, it is reported that 'the throttle was adjusted'   -  this is puzzling, because a partial loss of power is highly likely due to icing and fiddling with throttle would be futile. Immediate application of full carby heat at the 1st sign of engine 'coughing' may have restored power.

 

2. FINDINGS

 

(a) disagree with their finding that carby ice was unlikely:

 

This fails the 2nd dot point of their own 'key' points ex the ATSBs own 'Avoidable Accidents No 3 - Managing Partial Power Loss after Takeoff. Here they talk in terms of conducting ground runs so as to avoid the risk of partial power loss - yet don't spell out what this includes. Any time spent idling on a holding point, whether in likely icing conditions or not, should be followed by application of carby heat before take-off.

 

(b) disagree with their failure to evaluate all the after take-off actions of this flight.  

 

Again, they have not referred to their own publications key points - in this case, dot point 1 re pre-flight planning. A complete pre-flight self brief by the PIC would have included what actions could be taken with respect to power loss after take-off. This, surely would have included immediate application of carby heat in the event of any engine faltering. This need not distract the PIC from flying the aircraft toward a possible landing site - it should be almost a reflex action. Much is made of 'throttle adjustments' - but apart from the friction nut slipping, what else could this achieve?

 

Notwithstanding my criticisms of ATSBs report, the young pilot involved did a good job of avoiding serious damage and injury under the circumstances. 

 

I do not believe that ATSB have seriously contributed anything to aviation safety via this report. Rather the opposite in fact.   The absence of a 'smoking gun' should not have prevented a closer look at many other possible contributing conditions and actions.

 

happy days,

 

 

[Thruster88]

If this was carb icing  there would have been at least 50 occurrences on that day

 

Mr piper makes no mention of removing ice before take off in the POH it simply says    carb heat - check

 

Power loss on take off check items, no1 fuel selector 

 

                                                                 no2 fuel pump on

 

                                                                 No 3 carb heat on,  and this may have more to do with providing  an alternative air supply to carb

 

Piper and Lycoming think that fuel flow is more likely reason for engine stoppage ?

 

 

[aro]

I would not expect carby ice to allow enough power to get airborne normally, then suddenly limit power to the point where it won't continue flying.

 

Looking at the map and the sequence of events in the report, it sounds like the pilot changed tanks very close to the time of taking off. My suspicion would be that there was some problem with tank selection, and the pilot found out how long it will fly on the fuel in the gascolator and fuel lines.

 

 

[poteroo]

If this was carb icing  there would have been at least 50 occurrences on that dayMr piper makes no mention of removing ice before take off in the POH it simply says    carb heat - check

 

Power loss on take off check items, no1 fuel selector 

 

                                                                 no2 fuel pump on

 

                                                                 No 3 carb heat on,  and this may have more to do with providing  an alternative air supply to carb

 

Piper and Lycoming think that fuel flow is more likely reason for engine stoppage ?

If so - why was the LAME unable to find any blockages, any water, any sediment, but adequate fuel in the selected (RH) tank?  If they were thinking it was a fuel flow issue - then the 'complete test' of the engines 'operation' should have detected any restrictions at high power, or, any intermittent functioning of either fuel pump, or both?

 

BTW, I have the power loss urgent actions beginning with Carby Air- HOT, because it's the most probable. But then I've not looked in a PA-28 POH for a very long time! Maybe I'm wrong?

 

happy days,

 

 

[poteroo]

I would not expect carby ice to allow enough power to get airborne normally, then suddenly limit power to the point where it won't continue flying.

In my experience, this does happen. It was only a partial power loss, as the pilot actually believed he could make it back to the airport - from 200 agl!

 

Looking at the map and the sequence of events in the report, it sounds like the pilot changed tanks very close to the time of taking off. My suspicion would be that there was some problem with tank selection, and the pilot found out how long it will fly on the fuel in the gascolator and fuel lines.

The timeline, (though only approx.) says 1st call at 1109, 1 min to run-up area, run-up completed and tanks changed, then taxied to holding point where cleared for to at 1114.  That means the RH tank was being drawn on for approx. 2-4 mins. Not a long time, but probably enough to detect a 'misalignment' of the tank selector?  Training aircraft are notable for wear & tear on the fuel selector, such that it might 'point' to the tank mark - but not have clicked into the 'detent' in the selector. (Our Aero Club had their C172N involved in a F/L due to this very problem - brought about by thousands of changes between tanks by pilots trying to balance the wings). If this was the cause, how was the misalignment 'missed' in the post-crash investigation?

 

Again, more questions, which don't appear to have been covered, (perhaps not reported?), in the investigation?

 

happy days,

 

 

[aro]

I was taught to do the run-ups on the tank to be used for takeoff, DO NOT CHANGE TANKS BETWEEN RUN-UP AND TAKEOFF. The theory is that the run-up proves the fuel supply is good.

 

The pilot assessed it as a partial power loss, but it's pretty clear that it was complete when you look at where it ended up. It may have taken a few seconds to lose it all, but there is no way there was significant power being produced.

 

The timeline isn't very clear., but the run-up is very close the runway and it doesn't suggest there was a delay at this point: "when the pilot informed ATC that he was ready to take-off, he was issued a takeoff clearance". I read it as 11:09 he was cleared to taxi to the run-up bay, he taxied there, did the run-up, then changed tanks which to me suggests much less than 4 minutes, probably less than 2 on the RH tank.

 

 

[poteroo]

I was taught to do the run-ups on the tank to be used for takeoff, DO NOT CHANGE TANKS BETWEEN RUN-UP AND TAKEOFF. The theory is that the run-up proves the fuel supply is good.The pilot assessed it as a partial power loss, but it's pretty clear that it was complete when you look at where it ended up. It may have taken a few seconds to lose it all, but there is no way there was significant power being produced.

 

The timeline isn't very clear., but the run-up is very close the runway and it doesn't suggest there was a delay at this point: "when the pilot informed ATC that he was ready to take-off, he was issued a takeoff clearance". I read it as 11:09 he was cleared to taxi to the run-up bay, he taxied there, did the run-up, then changed tanks which to me suggests much less than 4 minutes, probably less than 2 on the RH tank.

Correct re don't change tanks. If we run with the hypothesis of 'off position with the selector' - then somehow, or someone, changed the selector position - after the accident.  Otherwise, the LAME could not have conducted run-ups afterwards?  More possibilities?

 

 

[Thruster88]

If so - why was the LAME unable to find any blockages, any water, any sediment, but adequate fuel in the selected (RH) tank?  If they were thinking it was a fuel flow issue - then the 'complete test' of the engines 'operation' should have detected any restrictions at high power, or, any intermittent functioning of either fuel pump, or both?

BTW, I have the power loss urgent actions beginning with Carby Air- HOT, because it's the most probable. But then I've not looked in a PA-28 POH for a very long time! Maybe I'm wrong?

 

happy days,

AO 2016-082 another PA28 no faults found on three occasions !

 

 

[facthunter]

 PROVING a tank,( Function and selection) is a very valid point. If the selector is to "off" generally the plane can just get airborne, before the engine quits. Changing fuel configuration just before take off is NOT advised.. IF it has been running on that selection it has proven some things( Not ALL) It doesn't prove the fuel flow is adequate, for the engine on high power and there's no assurance of sufficient quantity unless you have some other way of checking THAT. A Gauge alone is not enough. It could have failed in any position. "Sight" ones can be very reliable on deep tanks and allowing for pitch attitude.

 

   Carburetter icing can be progressive and does several things. Change mixture and/or reduce mass airflow. Even on days where carb icing is NOT likely the "function" check is a rev drop. (Nothing else) That proves the mechanism is hooked up and the alternative" hot" air is being applied.

 

     IF you suspect icing may be present, a totally different approach would be warranted, which would entail a longer application of heat and some notice of what response you got and  result in the engine running normally. Perhaps a full throttle check on brakes would be warranted, prior to rolling. It's also required the engines performance during the take off roll be observed on any takeoff. If the engine doesn't reach normal revs . torque  or whatever you register  the performance with, you should abort the take off, and do further checks before flight.. Nev

 

 

[Yenn]

On the only occasion that I have had carbie ice on takeoff, I got to just below take off speed and the power just died. That was in a cold humid environment. Not a problem, full heat and it eventually ran OK, but by that time the take off had been aborted and I taxied back to talk to my instructor.

 

I have read that take off is the most probable time for carbie ice and I do not agree with that. My opinion is that reduced power on a hot humid day is the most likely condition. What I do agree with is that carbie ice at take off is a very serious problem.

 

 

[facthunter]

 The latent heat of evaporation of the fuel is a major contributor. The most fuel by far is the rich mixture full throttle at take off, but the pressure drop in the venturi area and down stream of the butterfly when part closed or closed are also contributors. At CLOSED, not much airflow is happening, so I can't see things happening quickly. Your muffler and engine do cool off though so the de icing heat source opportunity is diminishing

 

       COLD ambient is not generally as humid in "absolute" water content  measurements. A 100% relative humidity at say 12 degrees will be nowhere near as much  ACTUAL moisture as the 100% RL at say 28 degrees. Some ice will form when ever the water vapour condenses and  it's below freezing in the area  being considered. so we are considering the potential RATE of ice formation. IF the carburetter body is below freezing the ice will bond with/stick to it, so will continue to build up whilever the moisture and temp continue to remain conducive for it. or till you take appropriate action.. Nev

 

 

[Jaba-who]

I agree. Winter is generally far less humid than other seasons in temperate climates. In other latitudes I am not so sure but Winter in the Tropics is also the dry season so presumable less humid as well.

Not quite. 

In the tropics the dry season is  late autumn/early summer. Starts about August but recent  years doesn’t kick in till October November.

 

November onwards we used to get afternoon thunderstorms but it’s become later and later. Now  It’s dry till often January and more recently February. Then the wet season kicks in. All this is a bit variable but that’s what we have had now for maybe a decade  or more. 

 

Coastal tropics in Australia we get trade winds starting in mid winter to August and they frequently bring south easterly stream rain. July it usually rains in the last half and can set in for a couple of weeks. Usually coincides with our show week here in Cairns.