Thursday, November 06, 2008

JungleBus Systems Post: Ice Protection

I recently had a passenger stop at the flight deck during deplaning to say hi to me and the FO. It turned out that he's a Private Pilot with an instrument rating, and flies a nearly-new Cessna 172 with a Garmin G1000 glass cockpit, traffic information system, and weather datalink. We marveled at the advances in general aviation avionics the past few years, and I remarked to him that his cockpit was as sophisticated as ours for a mere fraction of the cost. "Yeah," he said wistfully, "but it's still a single-engine piston, and despite all the goodies I still can't fly it half the winter."

It's a good point. Advanced avionics have brought light aircraft up to transport category standards in many respects and greatly improved their usefulness, but icing remains a significant problem for light aircraft. Despite some advances in technology to make anti-ice equipment lighter and cheaper, only some light twins and very few singles are approved for flight into known icing. A lot of the equipment is of dubious effectiveness, and prudent pilots don't remain in icing conditions for long even in "known ice" airplanes. Transport category jets, on the other hand, have had icing pretty well licked for over 40 years. This is in large part due to their superior performance: icing conditions tend to be pretty localized both by area and altitude, so aircraft with plenty of speed and power can blow through icing too quick for it to be much of a problem. Icing at cruise altitudes is rare for jets; the air is usually too cold to support enough moisture for significant icing. Jets have a significant advantage in the equipment department, too. They enjoy a steady supply of hot air from their engines' bleed valves, which is used to heat the wings, tail, and engine inlets. It's been an effective system since its inception, and has been adapted on most jet aircraft from the DC-8 until today - although the B787 will soon be a notable exception.

The JungleBus is no exception. Its ice protection equipment is pretty standard for a jet, although the operation is more automated than most aircraft. The leading edges of the wings are heated by bleed air via the pneumatic system; the engine intakes are heated by air that comes directly from the 10th stage compressor bleed. Both vertical and horizontal stabilizers are unheated; this is noteworthy but not unprecedented. The manufacturer had to prove during certification tests that the aircraft was not prone to tailplane stall or control problems with unusually heavy ice accumulation on the tail. I know - I'm not utterly convinced, either. The remainder of the anti-ice system is electric; protected areas include both windshields, the Air Data Smart Probes (pitot/static/AOA) and True Air Temperature (TAT) probes.

A key difference between hot-wing anti-ice systems on jets and the inflatable rubber de-ice boots used on smaller aircraft is when they should be turned on. Despite some controversy on the subject, most pilots still wait to inflate their de-ice boots until there has been some accumulation of ice. Anti-ice systems, however, must be turned on at the first sign of icing (or before). You don't want ice to build up on the cowl inlet only to be ingested into the engine when the cowl is heated; it can do a lot of damage to the compressor blades. Likewise, applying heat to a wing leading edge that already has a significant accumulation of ice can cause it to melt and refreeze further aft on the unprotected portion of the wing. Because it's hard to see the wing tips on most swept-wing airplanes, you have to rely on other cues to know when you've started accumulating ice. The windshield wiper often accumulates ice before any other part and makes for a good visual first warning. Transport category aircraft are also required to be equipped with ice detectors. These ingenious devices are metal rods that protrude from the nose of the aircraft which are vibrated at a particular frequency. Any ice accumulation will change the frequency of the vibrations, triggering an icing warning in the cockpit. Occasionally the detectors are heated to knock off existing ice so they can determine whether icing conditions still exist.

On most aircraft, the various icing systems must be turned on manually. Most operators direct their pilots to do so when entering potential icing conditions (clouds or visible moisture near or below freezing temps), or at the latest when the ice detector gives an icing indication. This is where the JungleBus departs significantly from previous designs; it makes normal operation of all anti-ice systems fully automatic. The ice protection panel is a collection of dusty switches that rarely get touched; so long as the mode selector remains in AUTO, the system will automatically turn on wing and engine anti-ice whenever the ice detectors sense icing conditions from shortly after takeoff until landing. Meanwhile the windshields are protected any time there are at least two sources of AC electrical power, and the probes are heated automatically whenever an engine is running or manually via a button on the FO's main panel.

There is some manual control of the system for abnormal operations and for takeoff. Although the mode selector is normally left on AUTO, turning it to ON manually activates engine and wing anti-ice (on engine start and liftoff, respectively). Individual selector buttons for the wings, each engine, and each windshield allow each component to be manually deactivated. These are seldom used except when components fail. Manual control of anti-ice systems for takeoff via the Flight Management System is much more common. Using bleed air robs the engine of compressed air for combustion and therefore use of wing and engine anti-ice results in decreased power output. For this reason, the JungleBus inhibits automatic operation of both wing and engine anti-ice until reaching 1700 feet AGL after takeoff. If anti-ice protection is desired for takeoff, the pilots must set it on the Takeoff Dataset Page of the Multi-Function Control Display Unit (MCDU, otherwise known as the FMS head). The standard mode, which inhibits anti-ice until 1700 feet AGL, is OFF. Turning it to ENG mode turns on engine anti-ice as soon as the engine is started. The ALL mode enables engine anti-ice on engine start and wing anti-ice when wheel speed exceeds 40 knots on takeoff. In both cases, the anti-ice systems revert to automatic operation once the plane reaches 1700 feet AGL.

At NewCo we turn the dataset to ENG mode if there is any precipitation falling or any surface contamination with a static air temperature of less than 10 degrees C, and we use ALL mode if there is visible moisture below 1700' AGL with a SAT of less than 5 degrees C. Of course we won't get any wing protection until achieving 40 knots on takeoff, so any prior contamination must be removed with de-ice fluid (Type I). This provides limited protection for active icing conditions on the ground (ie falling snow) so in many cases we follow application of de-ice fluid with a second coat of anti-ice fluid (Type IV). This is designed to absorb the falling precipitation and then shear off the wing during the takeoff roll, at which point the aircraft's own anti-ice systems will be protecting it.

Like most aircraft systems on the JungleBus, there is a dedicated synoptic page for the anti-ice system on the Multi Function Display (MFD). It displays the status of the bleed and anti-ice valves, pneumatic system pressures, and bleed air & wing duct temperatures. A color-coded schematic of the system makes it easier to quickly understand any abnormal conditions.

One annoyance of the JungleBus' ice protection system is that the ice detectors interact with the Stall Protection System with no provision for pilot intervention. Once ice is detected, the SPS will assume it remains on the airframe for the remainder of the flight and increase stick shaker & pusher speeds accordingly. This forces the pilots to use faster approach and landing speeds, which would be the correct thing to do anyways if the aircraft was actually loaded up with ice. It is, however, pretty ridiculous to be forced into using ice speeds to land in Dallas on a 90 degree day just because you picked up a trace of ice on climbout from Minneapolis several hours ago.

All in all, though, the JungleBus' ice protection system works pretty well. Last winter I had a few occasions where the ice built up pretty good on the windshield wiper before landing, and post-flight inspection revealed significant accumulation on unprotected surfaces but the heated portions of the wings and cowls remained absolutely clean. The aircraft handled quite well despite having a decent amount of ice on the nose, wing roots, and (gulp) tail. Really, the JungleBus' best ice-fighting technology is its thrust-to-weight ratio. Although transport category aircraft do have better equipment for dealing with ice than light aircraft, prudent airline pilots use it the same way as prudent private pilots: to keep ice accretion to minimum while exiting icing conditions ASAP. Being able to climb rapidly through icing layers means that the JungleBus' good ice protection comes in handy primarily when you find yourself stuck at a bad icing altitude for several minutes on approach.

We're supposed to get snow tonight so I think it won't be long before I put this knowledge to practical use. In my next post, I'll delve more deeply into de-icing ground procedures.


4 comments:

phil said...

interesting

Tim said...

I really am enjoying these Systems posts to compare the JungleBus with the Boeing and Airbus models I am more familiar with, and just to learn about the technology used.

Anonymous said...

Great reading - enabled by excellent writing.

Thanks for the technical, plus experiential, treatise.

Glad to know that these systems and capabilities are in place. Gives me confidence as a passenger and armchair captain.

John W said...

Glad you brought this up Sam. It was discussed recently on the A-net Tech Ops forum. Your post sheds somelight on the Jungle Bus ice protection. Perhaps Embraer will re-think how the ice and stall systems interact, like an override for the flight crew.