The accuracy and timeliness of environmental forecasts in support of aviation can be directly related to the success of airborne operations. This includes not only the aircraft themselves, but the sensors they employ and the conditions prevalent during maintenance. Navy Mobile Construction Battalion detachments also depend on forecasts and appropriate warnings for success of projects. This section will focus on these environmental effects and how they influence local operations.
1. ICING. Icing is a major winter hazard to flight. With a low freezing level and dense cloud cover in the modified continental polar air masses in winter it can become a major factor in local operations, and, it can become an important hazard to aircraft enroute to and from Souda Bay. Icing conditions, including freezing precipitation associated with frontal systems, are not uncommon over Europe during the winter.
a. Factors Affecting Icing.
Many factors affect the amount and rate of ice accumulation on an aircraft.
These include temperature, liquid-water content of the cloud, droplet size,
collection efficiency, and the aerodynamic heating of the aircraft's surfaces.
Unfortunately, temperature is the only parameter readily available with
which to forecast icing conditions. Cloud cover extent can be determined
with both visual and IR satellite imagery and surface observations from
b. Air Temperatures. The temperature range that ice is most likely to occur is from +2 to -20 degrees Celsius, and to a lesser extent from -20 to -40 degrees Celsius. Icing is rare in temperatures colder than -30C, and most icing is found at altitudes below 30,000 feet.
c. Cloud Types.
In stratiform clouds at middle and high levels, icing is generally confined
to a layer between 3,000 and 4,000 feet thick, and continuous icing conditions
rarely exceed a depth of 6,000 feet in stratiform clouds.
(2) In cumuliform clouds, the icing extends greater vertically than in stratiform clouds, but less horizontally. Icing conditions in cumuliform clouds range from light, in small supercooled cumulus, to moderate or severe in cumulus congestus and cumulonimbus clouds.
(3) Aircraft icing rarely occurs in cirriform clouds except in dense cirrus and anvil tops of cumulonimbus where conditions of up to moderate icing have been encountered.
d. De-icing/anti-icing. The following is provided as a guide to the forecaster on the availability and quality of aircraft deicing/anti-icing equipment:
C-130:Fair to good anti-icing/de-icing gear,
(2) C-12: Good to outstanding anti-icing/de-icing gear,
(3) C-9: Good to outstanding anti-icing/de-icing gear,
(4) P-3: Good to outstanding anti-icing/de-icing gear (both on leading edges and engines).
(5) H-2: No anti-icing equipment,
(6) H-53: Poor to fair de-icing gear.
e. Icing hazards on aircraft. Icing seriously impairs aircraft engine performance, operation of control surfaces, and the airfoil shape of wing surfaces. Specific hazards and effects on operations depend on the type of aircraft and systems. Four general types of aircraft and effects of icing are discussed below:
(1) Reciprocating Engine Aircraft. This type of aircraft generally operates for long periods at altitudes where icing conditions are most often found. Slower speeds also result in a lesser amount of aerodynamic heating of the aircraft surfaces,
(a) Propeller icing causes a tremendous loss of power and vibration. Modern
propellers have deicers, but these are not preventive devices, and the
danger still exists,
(b) Carburetor icing is a very serious hazard and can result in engine failure. This ice is caused by the temperature drop in the carburetor of as much as 40 degrees Celsius but usually 20 degrees Celsius or less.
(2) Turbojet Aircraft.
(a) Structural ice is usually not of much concern to turbojet aircraft
operating at high altitudes. Hazards exist, however, during landings, takeoffs,
climbs, and when operating at slow speeds at low altitudes,
(b) Induction icing. Besides structural icing hazards, internal icing in the air intake ducts may cause a hazard. This type of icing may occur in flights through supercooled water droplets in the same manner that wing icing occurs. It may also occur from lowered temperatures caused by low pressure in the intake systems during taxiing, takeoff and climb. Generally, if the free air temperature is 10 degrees Celsius or less and the relative humidity is high, the possibility of induction icing definitely exists.
Turboprop Aircraft. These aircraft engines are a combination of turbojet
and conventional, so the icing hazards combine hazards of both a and b
(4) Rotary Wing Aircraft. Icing conditions on helicopters affect the main rotor blades, the tail rotor, control rods and links, air intakes, and filters. Icing conditions for helicopters occur either in IFR conditions or in areas of freezinq rain and drizzle, and present a very serious hazard.
f. Intensities of Icing:
(1) TRACE - (very thin layer of ice)
(a) Not too significant to aircraft operations,
(b) Should be considered in clouds with temperatures between -22 and -40 degrees Celsius (cirrus level clouds).
(2) LIGHT - may be found:
(a) In clouds within 300NM ahead of warm fronts,
(b) With NO steady precipitation (i.e., intermittent) with temperatures from 0 to -15 degrees Celsius.
(3) MODERATE - may be found:
(a) Up to 100NM behind the cold frontal position,
(b) Over deep, almost vertical low pressure systems,
(c) In freezing drizzle.
(4) SEVERE - may be found:
(a) In freezing rain,
(b) In freezing drizzle if rate of fall is great enough (heavy freezing drizzle),
(c) In or near mature thunderstorms,
(d) In vicinity of jet streams - approximately 50-100 miles towards the low pressure side.
2. TURBULENCE/WIND SHEAR. The primary cause of turbulence is irregular movement of air in the atmosphere that causes eddy currents and wind gusts.
a. Effects of turbulence on aircraft:
Turbulence is directly proportional to speed of aircraft. Each type aircraft
has an optimum speed for penetration of turbulence,
(2) Turbulence is directly proportional to wing area,
(3) Turbulence is directly proportional to weight of aircraft.
b. Turbulence types and associated wind speeds/shears:
HORIZONTAL SHEAR VERTICAL SHEAR
LIGHT ----- 3-5KTS/1000ft
MODERATE 25-49kts/90NM 6-9kts/1000ft
SEVERE 50-89kts/90NM 10-15kts/1000ft
EXTREME 90kts+/90NM 15kts+/1000ft
NOTE: When two criteria are present in the same region, select the high intensity.
c. For turbulence reporting
criteria, see Figure IV-1
d. Occurrence of Intensities.
(1) EXTREME Turbulence:
(a) When encountered, which is rare, extreme turbulence is usually the
strongest form of convection and wind
(b) Most frequent locations:
1 In mountain waves or near rotor clouds,
2 In severe thunderstorms, especially those along squall lines.
(2) SEVERE Turbulence may be found:
(a) Up to 150 miles leeward of a ridge associated with mounain waves,
(b) In or near mature thunderstorms,
(c) In vicinity of jet streams - approximately 50-100 miles towards the low pressure side.
(3) MODERATE Turbulence may be found:
(a) Up to 300 miles leeward of a ridge associated with mountain waves,
(b) In TCU and thunderstorms,
(c) In vicinity of jet stream,
(d) At low levels when surface wind exceeds 25 knots (gusts or sustained).
(4) LIGHT Turbulence may be found:
(a) In mountainous areas even with light winds,
(b) In and near CU clouds,
(c) Near the tropopause,
(d) At low levels with surface winds greater than 15 knots,
(e) On hot, sunny days when superadiabatic conditions exist.
e. Low Level Wind Shear (LLWS) - Wind Shear is defined as a significant difference in wind direction and/or speed with distance/height. The most significant area of LLWS occurs between the surface and 2000 feet. LLWS causes changes in aircraft performance, which can result in a loss of altitude and possible aircraft mishaps. If forewarned of its presence, pilots may be able to compensate for its effects on landings and takeoffs:
(1) Indicated Air Speed (IAS) is a primary indicator of aircraft performance, (i.e. higher IAS = greater performance) and is affected by LLWS in the following ways:
(a) Change in wind direction:
TW to HW = +IAS
HW to TW = -IAS
(b) Change in wind speed:
>TW to <TW = +IAS
>HW to <HW = -IAS
Where: TW = Tailwind, HW = Headwind, +IAS = Increased IAS, - IAS = Decreased IAS
LLWS can result in a loss of lift on an aircraft wing. Since many aircraft
require up to 4 minutes to compensate for
a change in performance, significant LLWS can produce aircraft accidents
(3) Turbulence may or may not be present with LLWS.
(4) Conditions favorable for LLWS:
(a) Thunderstorm Gust Front. Operational impact depends on positions and
movement of the storm with respect to the
airdrome and the path of the aircraft. The gust front often precedes a storm by 5-10 miles, and with large storms may be 150 miles from the storm. A horizontal shear of 40kts/lNM is the required qust front with LLWS.
(b) Warm Fronts. Due to the slow movement and shallow slopes of warm fronts, LLWS may exist in one location for 6 hours or more. LLWS can be more dangerous near warm fronts than cold fronts because of both longer duration and it is usually unexpected.
Conditions for development:
1. A temperature gradient of 10 degrees Fahrenheit
or greater over 50NM,
2. Winds at-2000 feet of 40 knots or greater in the warm air.
(c) Cold Fronts. Due to fast movement and steeper slope, LLWS will occur for a shorter period of time (1-3 hours average) but will usually be stronger than LLWS found in a warm front. Conditions for development:
1 A temperature gradient of 10 degrees Fahrenheit
or greater over 50NM,
2 A vector wind difference across the front of 20 knots or more per 50NM,
3 Surface wind directional change of 50 degrees or more across the front (i.e., the wind will shift 50 degrees with
4 Frontal movement of 30 knots or greater.
(d) Gusty Surface Winds.
Local terrain features and buildings near the airfield can cause LLWS.
The resultant shear may occur within a few hundred feet of the surface,
when aircraft are most vulnerable.
(e) Land/Sea Breeze. Land/sea breeze depth is approximately 2000 feet. Effects of wind direction/speed change from
land/sea breezes can affect areas up to 100 miles inland.
(f) Low Level Jet. The low level jet core is normally just above the top of the inversion layer, causing large differences
in wind direction and speed in tens of feet. This condition causes the most dangerous LLWS with lower inversions (e.g., in the evening as the inversion begins to develop), because aircraft do not have as much time to compensate before landing.
3. THUNDERSTORM FLIGHT HAZARDS.
a. General. Thunderstorms
are local storms invariably produced by cumulonimbus clouds, always accompanied
by lightning and thunder, usually with strong gusts of wind, heavy rain,
and sometimes with hail at the surface and aloft. Within the thunderstorm
cell, there exists turbulence, sustained updrafts and downdrafts, usually
adjacent to one another in developing and mature stages, precipitation,
lightning and icing . All of these conditions present a threat to the safety
b. Turbulence. The chance of severe or extreme turbulence within thunderstorms is greatest at higher altitudes (between 8,000 and 15,000 feet AGL). The least turbulence may be expected when flying at or just below the base of the main thunderstorm cloud over relatively flat terrain. However, at low levels, low level wind shear caused by a gust front can cause rapid and drastic changes in low level wind direction and speed. Turbulence is greatest during the mature stage, when violent updrafts and downdrafts exist. Such conditions can cause structural damage to aircraft attempting to penetrate the storm. The heaviest turbulence is closely related to the areas of heaviest rain.
c. Icing. Severe icing should be expected in all thunderstorms. The most severe icing occurs in cumulus congestus clouds just prior to their change to cumulonimbus. Icing occurs at all altitudes above the freezing level in a building cumulus, but is most intense in the upper half of the cloud. An abundance of supercooled water droplets that occur in a cumulonimbus cloud in the layers between 0 and -15 degrees Celsius makes this area extremely hazardous, with rapid accumulations of clear ice.
d. Hail. This type of precipitation is regarded as one of the worst hazards of thunderstorm flying. It usually occurs during the mature stage of cells with an updraft of more than average intensity. Generally the larger the storm, the more likely it is to have hail. It is thought that most mid-latitude thunderstorms contain hail, although it often melts before reaching the ground. Hail may be encountered inside a thunderstorm and in the vicinity of a cell aloft. A strong updraft may send hail out through the sides and tops of a cell through a "hail shaft" with strong upper level winds possibly carrying hail as much as 100 miles downwind. For this reason alone, thunderstorms should be given a wide berth by aircraft. On the ground and aloft, even relatively small hail can damage the skin of an aircraft, especially one travelling at relatively high velocities.
e. Lightning. The electrical discharges of lightning is considered to occur most frequently in the area between the 0 Celsius and -9 Celsius temperature levels. However, lightning discharges may occur in other areas as the storm develops. Lightning strikes on aircraft are powerful enough to rupture the fuselage, fuel tanks and damage communication and electronic navigation equipment. Because lightning may ignite fuel vapors and cause explosions, the U. S. Naval Support Activity secures aircraft refueling/defueling during Thunderstorm Condition I. Ordnance handling is also suspended at the U.S. Naval Support Activity and the MOMAG (Mobile Mine Assembly Group) Detachment during Thunderstorm Condition I.
4. CROSS WINDS. Strong winds perpendicular to the runway pose a serious safety hazard to aircraft taking off or landing. Cross wind limitations are imposed on each type of aircraft and are published in appropriate aircraft flight manuals. Cross wind limitations for aircraft are based on a dry runway surface and are computed perpendicular (90 degrees) to the active runway. The following limitations apply:
AIRCRAFT MAX CROSS WIND
a. C-130 35 Knots
b. C-9 30 Knots
c. C-12 25 Knots
d. P-3 35 Knots
e. H-2 none *
f. H-53 none*
* Helicopters are affected by cross winds mainly on taxi. Maximum depends on pilot judgment and aircraft limitations.
|Join the GlobalSecurity.org mailing list|