latent weather threats

Latent Weather Threats

KoruSafe 58 KoruSafe 59

Nick Daniels

Flight Data Analyst


!"#$"%&!"'#'%(%'#)*)&"+#,&&"(#&-#&!"#aircraft works on the principle of radio echoing.

It operates in the x-band producing energy at very

high frequency in the form of electromagnetic

pulses which are emitted from a �at plate antenna

mounted in the radome. The antenna scans left

to right over an angle of 180˚ with the pulses

being emitted at regular intervals during the scan.

When the electromagnetic pulses come in contact

with precipitation they are re�ected back to the

scanner. The direction, distance and intensity is

then calculated by a transceiver/receiver unit and

displayed to the crew. The tilt of the weather radar

antenna beam, stabilised automatically in pitch

and roll to compensate for the aircraft’s attitude,

can be controlled manually to point above and

below the horizon up to ±15˚. This allows the

antenna beam to be moved upwards to reduce

the radar returns from the ground or to scan

di�erent levels of the atmosphere ahead. If the tilt

is selected to too high or too low an angle some

weather activity, that might a�ect the aircraft on

the track ahead, may be missed.

The radar is nothing more than a precipitation

detector and re�ectivity of precipitation particles

varies considerably depending on the type of

particle. The echo returns are proportional to

droplet size and precipitation intensity. Droplets

that are too small will return no echo, whereas

heavy droplets will return the majority of radar

echoes. Wet hail, rain and wet snow are much more

re�ective than dry hail, ice crystals or dry snow.

Unfortunately, aircraft radars do not see frozen

precipitation as well as they see wet precipitation.

So thunderstorm tops, which are composed of

mostly low-re�ectivity precipitation particles, and

may comprise the majority of the total mass of a

cloud are not seen very well by aircraft radar.

Scale of re�ectivity is as follows

Most: wet hail

heavy rain

rain

wet snow

dry hail

dry snow

Least: ice crystals

Of the least re�ective, the following pose the

greatest risks.

Ice crystal icing

Icing conditions have generally referred to

conditions where super-cooled liquid drops

adhere to cold airframe surfaces. Ice crystals of

substantial quantities can be launched high into

the atmosphere by convective activity typical of

thunderstorms, squall lines and tropical cyclones.

These crystals do not build up on the airframe and

are essentially invisible to on-board weather radar

and ice detectors. Pilots

have reported observing

rain as the crystals rapidly

melt on contact with

heated windscreens.

Ice crystals, regardless of

size, will not adhere to a

cold airframe but they can

partially melt and stick to

relatively warm engine

surfaces.The ingestion of

very small ice crystals into

the core of the engine

causes them to melt as they

impact on warm internal

engine components. With

an increasing collection of

super-cooled liquid a thin !lm is produced over

parts of the engine enabling further capture of ice

crystals. Over time they aggregate and reduce the

internal temperature of the engine which can lead

to engine malfunctions. Glaciated conditions refer

to atmospheric conditions containing only ice

crystals, and no supercooled liquid. Whereas mixed

phase conditions refer to atmospheric conditions

which contain both ice crystals and supercooled

liquid. Engine power-loss and damage events

have occurred in both glaciated and mixed

phase conditions [1]. To the on-board weather

radar, small particles, such as ice crystals in high

concentrations near thunderstorms, are invisible

even though they may comprise the majority of

the total mass of a cloud. Satellite radar technology

has however been able to detect crystals smaller

than the lower limit of the on-board weather

radar. Above the freezing level, where icing can

occur in a convective cloud, large particles are

only found near the convective precipitation

core. Away from the convective precipitation core

small ice crystals exist. For this

reason, �ight in visible moisture

near deep convective weather,

even without radar returns, and

at temperatures below freezing, is

very likely to be in ice crystal icing

conditions. Any ice building up on

the inlet, fan, or spinner will likely

shed outward into the fan bypass

duct without causing a power loss.

So it is reasonable to conclude in

these power-loss events that ice

must have been building up in the

engine core.

A number of agencies, engine and

airframe manufacturers conducted

various studies [2] into the bearing of high

altitude ice crystals on aircraft operations.

They identi!ed at least 150 incidents since

1989 where ice crystals caused problems for

the �ights concerned. The events occurred in

environments that appeared benign to pilots,

including an absence of airframe icing and

only light turbulence. High altitude ice crystals

in convective weather are recognized to be

the cause of engine damage and power loss

including surge, stall, �ameout and rollback.

In addition, some examples of engine blade

damage have been recognised. Most of these

instances have occurred at altitudes greater than

22,000 feet where there is usually an absence of super

cooled liquid water, the typical cause of airframe

icing. Engine power-loss events have occurred in the

climb, cruise, and descent phases of �ight. But most

events occur during the descent phase because of

a combination of factors. The ambient temperature

must be below the freezing level for icing to occur

and therefore tends to occur at the higher altitudes

associated with the descent phase. And secondly

the engine has a lower tolerance to ice shedding at

idle power. Although icing at high power and high

altitude is possible due to the existence of high

concentrations of ice crystals for long distances as ice

can build up on warm engine surfaces.

Over 60 per cent of these events have occurred in

Southeast Asia and Australasia. Researchers speculate

that this may be due to the fact that the highest sea

surface temperatures are also found in this region

with higher temperature air holding more water.


There is a concentration of ice crystal power-loss

events between 20 and 40 degrees north latitude

with a few events farther than 45 degrees from

the equator. SIGMETs and SIGWX forecasts do

not currently provide speci!c information about

ice crystal icing. But the inclusion of information

concerning thunderstorms, especially in tropical

areas, can provide pilots with an indication of the

potential risks of encountering areas of ice crystals

at high altitude.

Recognising Ice Crystal Icing Conditions

There are several identi!ed conditions that are

connected to engine ice crystal icing events. The

most important of which are.

High altitudes and cold temperatures. Aircraft

power-loss events associated with ice crystals

have occurred at altitudes of 9,000 to 39,000 feet,

with a median of 26,800 feet [3] and at ambient

temperatures of -5 to -55 degrees C with a median

of -27 degrees C.

The presence of convective clouds. Many types

of convective weather contain ice crystals.

Convective clouds can contain deep updraft cores

that can lift high water concentrations to high

levels in the atmosphere, during which water

vapour is continually condensed and frozen as the

temperature drops. These updraft cores

can produce high ice water content

regions which move downwind. These

clouds can contain up to 8 grams per

cubic meter of ice water content and

the supercooled liquid water design

standard for engines is 2 grams per

cubic meter.

Areas of visible moisture above the

altitudes typically associated with

icing conditions. This is indicated by

an absence of signi!cant airframe

icing and no sensing of ice by the ice

detector, due to its ability to detect only

supercooled liquid and not ice crystals.

These additional situations are also

typically been found during engine ice

crystal icing power-loss events.

• No pilot reports of weather radar

returns at the event location.

• Temperature signi!cantly warmer than standard

atmosphere.

• Light-to-moderate turbulence.

• Areas of heavy rain below the freezing level.

• The appearance of precipitation on heated

windshield, often reported as rain, due to tiny ice

crystals melting.

• Airplane total air temperature (TAT) anomaly-

reading zero, or in error, due to ice crystal build-up

at the sensing element

• Lack of observations of signi!cant airframe icing.

What the Hail?

Hail represents a major threat, because of its e�ect

and the fact the weather radar does not indicate

the nature of returns due to its poor re�ectivity.

Only the knowledge of a Cb’s structure and the

observation of di�erent clues can help. The

presence of hail within a Cb, varies with altitude

and wind. Usually, the threat of hail is greater

downwind of a Cb as moisture is driven upward

by strong drafts. It then freezes and is transformed

into hail, before being blown downwind. When

possible, it is better to try to avoid a Cb by �ying

on its upwind side. There is less risk of hail in

humid air than in dry air. In fact, moisture in the

air behaves as a heat conductor, and helps to melt

the hail. Hail is formed by collision when drops of

water freeze together in the cold upper regions

of thunder storm clouds. These drops are liquid

drops surrounded by air that is below freezing

which is a common occurrence in thunderstorms.

Most hail measures between 5mm and 15cm in

diameter, and can be round or jagged.

There are two methods by which a hailstone

grows, wet growth and dry growth, which produce

the layered look of hail.

In wet growth, the hailstone nucleus, a tiny piece

of ice, is in a region where the air temperature is

below freezing, but not super cold. Upon colliding

with a supercooled drop the water does not

immediately freeze around the nucleus. Since the

process is slow, air bubbles can escape resulting in

a layer of clear ice.

With dry growth, the air temperature is well below

freezing and the water droplet immediately freezes

as it collides with the nucleus. The air bubbles are

frozen in place, leaving cloudy ice. It is dry hail that

re�ects poorly on the airborne weather radar.

Strong updrafts can create a rain-free area in

supercell thunderstorms. Meteorologists call this

area a WER which stands for “weak echo region”. It

is bounded on one side and above by very intense

precipitation indicted by a strong echo on radar.

This rain-free region is produced by the updraft

and is what suspends rain and hail aloft.

1. The hail nucleus, buoyed by the updraft is carried

aloft and begins to grow in size as it collides with

supercooled raindrops and other small pieces of

hail.

2. Sometimes the hailstone is blown out of the

main updraft and begins to fall to the earth.

3. If the updraft is strong enough it will move

the hailstone back into the cloud where it once

again collides with water and hail and grows. This

process may be repeated several times.

4. In all cases, when the hailstone can no longer be

supported by the updraft it falls to the earth. The

stronger the updraft, the larger the hailstones that

can be produced by the thunderstorm.

Multi-cell thunderstorms produce many hail

storms but usually not the largest hailstones

because the mature stage in the life cycle of the

multi-cell is relatively short which decreases the

time for growth.

However, the sustained updraft in supercell

thunderstorms support large hail formation by

repeatedly lifting the hailstones into the very

cold air at the top of the thunderstorm cloud.

The stronger the updraft the larger the hailstone

can grow. In all cases, the hail falls when the

thunderstorm’s updraft can no longer support the

weight of the ice.

The table provides the approximate speed for

each size. How strong does the updraft need to be

for the various sizes of hail.

Anyone who’s ever ridden a motorcycle can attest

that even rain drops at 100 kph are painful, imagine

chunks of ice while travelling at over 900 kph. The

fact that the windows were cracked but structurally

intact is a testament to the engineering which

goes into their design. A complete windscreen

failure could cause a serious event. Windscreens

are designed to take a lot more abuse than hail

can create as their design speci!cations include

hitting a large bird at a very high speed. The

windscreens are a multi-layered laminate which

includes a heating layer embedded in the glass to

keep it from becoming brittle. Even if one of the

layers appears shattered, the load bearing layer

most likely is not. The biggest challenge the pilots

Hailstone size Measurement Updraft Speed

in. cm. mph knots

bb < 1/4 < 0.64 < 24 < 21

pea 1/4 0.64 24 21

marble 1/2 1.3 35 30

dime 2/3 1.8 38 33

penny 3/4 1.9 40 35

nickel 7/8 2.2 46 40

quarter 1 2.5 49 43

half dollar 1 1/4 3.2 54 47

walnut 1 1/2 3.8 60 52

golf ball 1 3/4 4.4 64 56

hen egg 2 5.1 69 60

tennis ball 2 1/2 6.4 77 67

baseball 2 3/4 7 81 70

tea cup 3 7.6 84 73

grapefruit 4 10.1 98 85

softball 4 1/2 11.4 103 90

An idealized path of hail within cloud.


might have faced after the hail !rst hit, would have

been landing with an obscured view through the

damaged windscreen and autoland capability

may not have been possible. It is also interesting

to note that, according to the manufacturer, if the

radome had become detached from the aircraft,

the fuel burn, resulting from the increased drag,

could have been increased by as much as 27%.

Recommendations for �ight near convection

Even when there are no radar returns, there may

be signi!cant moisture in the form of ice crystals.

There may also be hail present. These types of

precipitation are not visible to airborne radar so

it is not always possible to avoid these conditions.

Normal thunderstorm avoidance procedures

may help pilots avoid regions of high ice crystal

content and/or hail.

These avoidance procedures include:

• Avoiding �ying in visible moisture over storm

cells. Visible moisture at high altitude must be

considered a threat since intense storm cells may

produce high concentrations of ice crystals at

cruise altitude.

• Flying upwind of storms when possible.

• Using the radar antenna tilt function to scan the

re�ectivity of storms ahead. Assess the height

of the storms. Recognize that heavy rain below

the freezing level can typically indicate high

Airbus A321-200 (PT-XFB), operating TAM Linhas Aéreas �ight JJ3307 from Rio de Janeiro–Galeão,

Brazil to Fortaleza, Brazil, turned back to land at Rio de Janeiro–Galeão sustaining damage in a hail

storm.

Pilots on a �ight in the US were left �ying blind after their plane hit hail storm that shattered the

windscreen.

concentrations of ice crystals above.

• Avoiding storm re�ectivity by 20 nautical

miles has been commonly used as a recommended

distance from convection. Again this may not be

su$cient for avoidance of high concentrations of

ice crystals or dry hail, as they are not visible on

airborne radar.

Reference List

[1] Mason, J. (2007). Engine Power Loss in Ice

Crystal Conditions. Aeromagazine QTR4.07,

Boeing, Seattle, Washington.

[2] Mason et al., (2006). Ice Particle Threat to Engines

in Flight. American Institute of Aeronautics and

Astronautics (AIAA) 44th AIAA Aerospace Sciences

Meeting and Exhibit, Nevada.

[3] Grzych, M., 2010. Avoiding Convective

Weather Linked to Ice-Crystal Icing Engine

Events. Aeromagazine QTR1.10, Boeing, Seattle,

Washington.

[4] John Werth Airborne Weather Radar Limitations

by, Seattle ARTCC Center Weather Service Unit

[5] Airbus, Flight Operations Brie!ng Notes

Adverse Weather Operations Optimum Use of the

Weather Radar

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