Jet airliner ice protection systems - a cautionary tale.

[Centaurus]

While reading an old 1999 United Airlines flight safety magazine, an article on airframe icing types caught my interest.

It said the ice protection system on big jets is essentially designed to handle rime icing conditions and that mixed and clear icing conditions can exceed the design limits of the aircraft ice protection system. Reason is that rime ice has relatively low levels of liquid water and as such the ice will confine itself to leading edges. The ice protection systems can remove or prevent it.

On the other hand, mixed and clear ice are associated with precipitation-size drops and relatively high quantities of liquid water. These conditions can deposit ice on and AFT of your leading edges and can flow back. When this happens your ice protection system cannot remove the aft portion of the ice because it was never designed to handle this type of ice.

Does this icing limitation apply to current jet airliners? And where GA light aircraft are equipped to fly in icing conditions, is that rime ice only - for the same reason?

If so, seems to me that a thorough study of the type of icing given in a weather forecast is vital to safe operation. I understand that the various regulations that govern flight into known icing conditions do not necessarily state the type of ice?

[alatriste]

As far as I know the only restriction is: DO NOT TAKE OFF OR LAND ON FREEZING RAIN.

WHY? because rain droplets are big enough to hit the airfoil not only at the leading edge but all over the wing. Airfolis are only protected in the LE.
If the water drops are smaller they will not hit the wing, just the leading edge, because the air flow over the wing will prevent the drops to get in contact with rear of the wing area.
Just think about the snow flakes when this snow is really dry, and not heavy cause of little water, when is snowing that way the flakes will never touch your car windscreen if the car is moving faster enough. If the car stops it will get wet by these flakes. On the other hand if it is raining, your windscreen will get wet regardless of your speed and the use of the wipers is a must.

[Mad (Flt) Scientist]

If so, seems to me that a thorough study of the type of icing given in a weather forecast is vital to safe operation. I understand that the various regulations that govern flight into known icing conditions do not necessarily state the type of ice?

Not quite true. Appendix C to 14 CFR 25 (FAR 25) does define various icing conditions which are to be considered in showing compliance with 25.1419, the ice protection requirement.

It is, however, now recognized that the icing conditions so defined do not cover cases such as "supercooled liquid droplets" (SLD) and there are, therefore, conditions where any ice protection system can be overwhelmed, either because of the nature of the icing (causing ice accumulation far back on the wing, as described) or simply because the icing conditions are so extreme that the anti-ice system cannot keep the leading edge clear as intended.

The best that can be done is to provide guidance to crews as to when they may be in extreme icing conditions and instruct them to get out as quickly as possible. This might be as a result of, say, observing ice buildup on a nominally protected surface or buildup in unusual locations.

[Clandestino]

No take off during freezing rain is because freezing rain dillutes type II/IV anti-icing fluid too quickly and there's not enough time to take off while you still have some protective coating on top of the wings/stabilizer. And if you haven't applied anti-icing you are not allowed to take-off during any freezing precipitation. Trying to override this rule has caused way too much grief in the past.

I don't think that jet transports have any problems with runback icing. Their high speeds make it highly unlikely and if you reduce speed in clean config, AoA goes quite noticeably up, and the only area exposed to potential runback icing is wings' lower surface - which is not as susceptible to icing effects as the upper one is. Bleed air heated wings' leading edges are not designed to cope just with "rime" icing, they'll happily handle any type of icing. Besides there are no "area of rime ice", "area of clear icing" etc. Different airplanes, flying at different speeds through same area will experience different types of icing. Only rate of ice accretion is forcasted/reported.

The infamous Roselawn crash was related to runback ice. ATR involved was flying in freezing rain conditions, with low speed and low AoA due to flaps been half down and ridge of ice formed behind de-icing boots. When crew retracted flaps in descent, this ridge disrupted airflow over the ailerons, moving them to full deflection and causing unrecoverable roll and dive. If they hadn't extended the flaps in holding, if they hadn't retracted them later, if the ailerons had been better designed, probably we wouldn't even know there's phenomenon called runback icing. ATRs later got wider boots and a lot of procedures for coping with flying in icing conditions.

[alf5071h]

As M(F)S indicates, the certification requirements are very limited.
The details are outlined in the following link with some very good general advice; 'begin the course', skip the intro and see Aircraft design for certification. (http://aircrafticing.grc.nasa.gov/courses.html#)

Some of the assumptions surrounding the certification and operation may be in other documents; the following extract I believe came from the UK AIP:
All pilots encountering un-forecast icing are requested to report time, location, level, intensity, icing type and aircraft type to the ATS unit with whom they are in radio contact with. It should be noted that the following icing intensity criteria are reporting definitions; they are not necessarily the same as forecasting definitions because reporting definitions are related to aircraft type and to the ice protection equipment installed and do not involve cloud characteristics. For similar reason, aircraft icing certification criteria might differ from reporting and/or forecasting criteria.

Icing Types
‘Rime Ice’ - Rough. milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.
‘Clear Ice’ - A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets.

Icing Intensity (accumulation)
‘Trace Ice’ - becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. It is not hazardous even though de-icing/anti-icing equipment is not utilized, unless encountered for more than one hour.”
‘Light’ - The rate of accumulation might create a problem if flight in this environment exceeds 1 hour. Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not present a problem if de-icing/anti-icing equipment is used.
‘Moderate’ - The rate of accumulation is such that even short encounters become potentially hazardous and use of de-icing/anti-icing equipment, or diversion is necessary.
‘Severe’ - The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
The aircraft AFM icing clearance generally relates to the certification definition. There are few if any absolute regulations or limitations relating to icing conditions, these are deemed operational issues and thus the source of many misinterpretations and errors.

Note that the ice accumulation definitions relate to the capability of aircraft’s de/anti-icing system; thus in-flight reports of icing conditions are most unreliable unless you know the capabilities of the other aircraft and how the crew are using their systems.

Edit: Also see In Flight Icing Encounter (www.fly-safely.org/story.asp?id=76)

[Centaurus]

Thank you all very much for the excellent replies. They are saved on my computer as hand-outs for 737 type rating courses.

[Grunf]

Centaurus,

you may find a lot of info on the net related to icing but the main two(and especially if you're in a GA plane) things are:

1) avoid flying when there is freezing rain since there is no anti icing design/fluid to keep you constantly ice free

2) Always do a through check-up of your upper surface prior to take-off.

I am referring particularly to item two since that killed a crew and passengers of Challenger in UK 2 or 3 years ago. Similar incident happened more recently in New Jersey (Teterboro) also with Challenger.

Since business a/c pilots (and GA pilots as well) are not obliged by the same rules as those flying on Part 121 (and similar) it depends on their judgment and pre-flight inspection to get ready to fly in bad weather.

Special concern goes for business and GA since they can go to any airfield possibly and probably without anti-icing fluid and spraying activity.

In that case you only depend on leading edge anti-icing system which (if not pneumatic) is powered by a bleed air from engine.

It won't help you to have a strong engine - drops on upper side of you airfoil will create so much instability and loss of airflow that you will be very quickly flying like a rock:ugh:

Cheers

[Centaurus]

Grunf. Thanks for info. I was aware that certain ice formations near the wing-tip on the 737-200, can cause a nasty pitch up and roll after rotation. This happened with the Potomac accident. It was worse if the take-off was with Flaps 1 set. A crusty old British check captain had a go at me for rotating (in his opinion) too slowly during Flap 1 take-offs. He said my slow rotation would compromise obstacle clearance. In fact I was rotating strictly according to Boeing recommendation - 3 degrees a second.

I had earlier noticed his own rotation rate was very fast but as he was my boss I kept quiet. Finally, over several sectors I quietly timed his rotation rate by stopwatch and it took 2 seconds from VR to 15 degrees body angle as he forced the aircraft hard through the dead spot that occurs around 10 degrees due the tail going into ground effect momentarily. He would not believe it. Stubborn, but quite likeable old bastard. Him - not me, of course!

[Grunf]

Huh,
I guess you've never sat on Northwest Airlines DC-9, I presume.

Pilot you're talking about seems like a mellow version for their take off procedure in similar conditions to yours.

I have never been in such a quick combination, 2 sec or more I don't know but it seems it is important to them to get as high as soon. Sort of afterburner effect, I guess, they want to achieve?!

A good approach for icing conditions though since all the experiences I had in Montreal which can get quite cold in winter (-27 Celsius + wind = -40 to -45 deg Celsius).

Freezing rain is still scary since myself and anyone who ever lived in Montreal (or anywhere in that latitude) knows how bad it can be when you drive. And to be in the a/c...wooo...