Icing in Flight and Contaminated Runways - ATPL 070 — Operational Procedures

What ice does in flight

The syllabus opens with a warning against tidy numbers: the effects of icing are wide-ranging, unpredictable and dependent on individual aircraft design. It then offers the magnitudes the exam quotes: in icing conditions, for a given speed and a given angle of attack, wing lift can be reduced by as much as 30 per cent and drag increased by up to 40 per cent, changes that eat cruise performance, climb gradient and the margin above the stall together. The acceptable means keeps the operator honest about the machinery: procedures name the equipment that must be serviceable for icing flight, the limitations for each phase, the criteria by which the crew assesses icing's effect on performance and controllability, the visual cues or ice detection systems that announce the conditions, and the actions for a deteriorating situation, whether the protection systems lose the race or ice grows on unprotected areas.

Two mechanical corollaries carry their own learning objectives. Ice on the critical surfaces and airframe may break away during take-off and be ingested into the engines, damaging fan and compressor blades, the airborne echo of the ground lesson's ingestion warning. And ice forming on pitot tubes, static ports or angle-of-attack vanes feeds the air data systems false altitude, airspeed, angle-of-attack and engine-power information: the aircraft may be flying honestly while its instruments lie.

Ground ice is not flight ice

The syllabus insists on a distinction easy to blur: ice, frost and snow formed on the critical surfaces on the ground can have a totally different effect on flight characteristics than ice accreted in flight. Ground deposits sit in shapes and places that in-flight icing does not produce, which is why the previous lesson's clean aircraft concept accepts no ground contamination at all while flight in icing can be certified. The housing question completes the set: flight in known icing conditions is subject to limitations contained in Part B of the operations manual, and the procedures and performance data for expected or actual icing live on the same shelf, the type-related Part B, alongside the aircraft flight manual it draws from: the answer the performance course gave three times over for contaminated operations, and the answer here too.

The runway condition code

The winter runway speaks in a number. The runway condition code, RWYCC, describes the effect of the surface on deceleration performance and lateral control, assessed by the aerodrome operator through the runway condition assessment matrix from the observed surface conditions and pilots' braking reports, and published in the runway condition report. The ladder pairs codes with braking-action reports: 6 is the dry runway with nothing to report, 5 is GOOD, braking deceleration and directional control normal, then good to medium 4, medium 3, medium to poor 2, poor 1, and 0, less than poor, braking minimal to non-existent, a report that, in the acceptable means, closes the runway until the aerodrome operator improves the condition.

The feedback rule is a commander's duty, rule-level: whenever the braking action encountered during the landing roll is not as good as reported in the runway condition report, the commander shall notify air traffic services by means of a special air-report as soon as practicable. The acceptable means adds the etiquette: standard terminology from good to less than poor, a coarser good-medium-poor scale acceptable when the shades blur, better-than-reported worth reporting too, and an air safety report whenever low braking action endangered the flight. The reports matter because the aerodrome operator uses them, through the matrix, to upgrade or downgrade the code the next arrival will plan on.

Hydroplaning

The performance course introduced hydroplaning where it bites the schedule; the syllabus asks this course for the same three definitions and the same arithmetic, and they must match. Dynamic hydroplaning has standing water lift the tyre bodily off the surface; viscous hydroplaning has a thin film on a smooth surface, painted markings or rubber deposits, lubricate the contact at much lower speeds; reverted rubber hydroplaning has a locked, skidding tyre boil the film beneath it and ride its own steam. The dynamic kind carries the formulas: with the tyre pressure P in psi, the spin-down speed of a rotating tyre is 9 times the square root of P, and the spin-up speed of a non-rotating tyre is 7.7 times the square root of P. A 121 psi tyre planes at about 99 kt rolling and about 85 kt unspun.

The exam's favourite subtlety is which speed governs when. On take-off the wheels are already rolling, protected up to the higher spin-down figure. At touchdown on a flooded runway the wheels arrive unspun, so it is the spin-up speed, the lower figure, that represents the actual tyre situation: an aeroplane can touch down above its non-rotating hydroplaning speed and find the wheels planing before they ever spin up, with braking and directional control gone until the speed decays.

Reading the SNOWTAM

The air-law course taught the envelope: the SNOWTAM is the special-series message in its own standard format carrying the surface condition report for the movement area, valid at most eight hours and replaced by each new runway condition report. Reading one is this lesson's skill. The message names the aerodrome and observation time, then works runway by runway: for each runway the runway condition code for each third, and for each third the coverage percentage, the contaminant depth in millimetres, and the condition description drawn from the defined descriptors, compacted snow, dry or wet snow, slush, standing water deeper than 3 mm by convention, frost, ice or wet ice. A situational awareness section closes the message with the taxiway and apron conditions, reported in the plain poor-good vocabulary, and remarks such as sanding, chemical treatment, snowbanks and their distance from the centreline, or drifting snow.

Read against the METAR, the message becomes a hazard list. Freezing rain in the present weather means a condition the previous lesson flagged as having no holdover guidance at all and a surface heading for wet ice; a low code on the last third is a warning about exactly where the landing roll ends; snowbanks in the remarks threaten wingtips and engine pods on a slippery centreline; a code of 2 or below with a gusting crosswind attacks the lateral control the code explicitly prices. The mitigations are the course's own toolkit: the landing performance assessment at time of arrival flown against the reported code, the operator's crosswind limits for that code, the deceleration devices scheduled as the performance course taught, patience or a diversion when the codes fall, and the special air-report that tells the truth back to the system when the runway under-delivers.

What ice does in flight

Ground ice is not flight ice

The runway condition code

Check you understood

Hydroplaning

Reading the SNOWTAM