View Full Version : Icing
26th May 2011, 04:23
This is a spillover from the Tech AF447 thread.
It is claimed the pitot on the A330 have passed all required tests, and clogging at FL 350 is outside the certification criteria.
In the distant past, I remember hearing how new airliners were tested for icing by following a plane that was spraying water while above the freezing level. What is the routine for icing tests? Can this not be done at flight levels?
Mad (Flt) Scientist
26th May 2011, 04:36
The spray tanker method isn't actually used very much (it may even be out of service now, certainly it was due to be retired at some point)
For something like a pitot, it would be more common to demonstrate AI performance in an icing tunnel, since you can go full scale and totally control the test conditions. Then you obviously also have the probes on the a/c when you do the airframe tests (in "natural icing") and that is used to confirm the tunnel data are valid. (No strange effects due to the presence of the airframe, for example)
The conditions are indeed outside the FAR 25 App C conditions. However, there are new specified conditions in the works, and aircraft moving through the cert process today can expect to have to meet newer, more challenging, rules for pitots etc. Initially it'll be by Issue paper or Cert memo or whatever the authority in question calls an ad-hoc rule (this is where the state of the art is today), and eventually it'll get into the basic regs and guidance.
26th May 2011, 04:51
Thanks for the info. How did the tanker disperse the water? Would the Evergreen 747 firefighting tanker with 23,000 gallons internal, or the DC-10 Tanker with 13,000 gallons external be suitable for such testing at flight levels?
Mad (Flt) Scientist
26th May 2011, 10:52
It was (is) a modified air-air refuelling tanker with a special spray bar attached to the refuelling boom. The smart bit was the spray bar, since it has to 9try to) give a controlled and known set of water droplets - just dumping a large amount of water in the air doesn't replicate an icing cloud.
IIRC the icing tanker could only reproduce certain ranges of droplet size, and maybe only certain sizes within that range on a given flight - I think they had interchangeable spray bars for different droplet sizes.
So any airframe with tanker capability would do, but only if fitted with the spray bar system.
26th May 2011, 21:12
GB, my experience is with TAT probe icing and engine malfunctions in similar conditions, but essentially these involve the same phenomena as the ‘pitot icing’ hypothesized in AF447.
As you are aware, this form of ‘icing’ is not conventional icing and is better described as ice particle icing, where very small particles accumulate in unheated parts of tubes, or freeze / refreeze where melting ice balances the heat flow of anti-icing devices.
The conditions where these very small particles are encountered were outside of the then normal icing certification envelope and thus did not have to be considered by manufacturers. Until relatively recently the science of icing did not recognize or predict such conditions.
The particle size, shape, composition are such that they are most unlikely to be reproduced by tanker aircraft. During the early investigations, the only facility capable of reproducing the conditions was at the Arnold Test Centre and involved liquid nitrogen in a wind tunnel.
The relatively ‘recent’ spate of problems from ice particle icing begs the operational questions why now? What has changed?
Some considerations, but not exhaustive:
Older aircraft did not have sufficient performance to reach the conditions (EASA - European Aviation Safety Agency (http://easa.europa.eu/iascc/doc/Workshop%201%20Presentations/Workshop1_DAY%202/1_Duvivier_EASA/IASCC_E%20Duvivier.pdf) slide 17), possible, and/or with a lower number of exposures, but perhaps the older systems (and engines) were more tolerant to these conditions.
Older aircraft did not fly in the conditions generally found close to large Cbs. Quite possible. Modern WXR provide crews with high quality information (but still not detecting ice); this could lead to a false sense of security, or overconfidence from routine operations. These, together with the pressure of modern operations / ATC / airspace restrictions could result in cutting Cb deviation too close.
Modern aircraft use sensors to provide (digital) data for many systems other than primary flight instruments, thus any malfunction (severe ice particle encounter) can have a wide ranging and noticeable effect.
27th May 2011, 19:12
G-B, a bit of `googling` under airborne icing tanker should get you further info,ie KC-135R-AIT,which is a USAF one; there is also a US Army CH-47D which has/had a bigger array for helos/GA aircraft ,and Cessna FTOps use a modified Citation 550.AAEE used to have a Canberra Bi8 modified with a spray bar in about `69-71 ?.I tested several helos over several winters in the NRCspray rig at Ottawa,before plunging into clouds. For an airborne tanker you should have a heated water supply,heated spray bars ,and heated and selectable droplet size nozzles,and a calibrated chase(ideally).
The water supply is also coloured with a dye marker..