jueves, 11 de febrero de 2010

Flight Safety Digest - Understanding the Stall-recovery Procedure For Turboprop Airplanes In Icing Conditions

Description

This article provides guidance for controllers on what to expect from an aircraft experiencing the effects of in-flight icing and some considerations which will enable the controller, not only to provide as much support as possible to the aircraft concerned, but also maintain the safety of other aircraft in the vicinity and of the service provision in general.

Useful to Know

In-flight icing poses serious threat to the safety of a flight. Water droplets accumulate on the airframe as ice under specific conditions. The ice deposits alter the wing profile and disrupt the flow of air. This drastically changes flight parameters such as lift, drag, controllability, etc. As drag increases, lift rapidly decreases – a natural pilot action to compensate this would be to apply power and increase angle of attack to maintain level, however this leads to even faster ice accumulation as larger airframe surface is exposed. Ice formations on wing and control surfaces lead to increased stall speed, sudden uncontrolled pitch or roll with eventual difficult recovery and loss of control.

Moderate or severe icing conditions could overwhelm the anti-ice system commonly fitted on modern aircraft. In this way, the safe continued flight could be impossible.

Some hazards associated with of in-flight icing include:

  • Adverse Aerodynamic Effects
  • Blockage of pitot tubes and static vents
  • Communications problems.

Type and speed of ice formation is a function of the variables of the cloud liquid water content, the mean effective diameter of the cloud droplets and the ambient air temperature. The following table shows the risk classification, depending on the cloud type and temperature:

In-flight icing risk Cumulus clouds Stratiform clouds Rain and drizzle
High 0° to -20°C 0° to –15°C 0°C and below
Medium 20° to –40°C -15° to –30°C
Low <><>
Figure 1: In-flight icing risk in various atmospheric conditions.

According to studies conducted on icing-related incidents and accidents, “the worst continuous icing conditions are found near the freezing level in heavy stratified clouds, or in rain, with icing possible up to 8,000 ft higher. Icing is rare above this higher altitude as the droplets in the clouds are already frozen. In cumuliform clouds with strong updrafts, however large water droplets may be carried to high altitudes and structural icing is possible up to very high altitudes. Further, in cumuliform cloud the freezing level may be distorted upwards in updrafts and downwards in downdrafts, often by many thousands of feet. This leads to the potential for severe icing to occur at almost any level.” (Aircraft Icing Handbook pp2, see Further Reading)

The following icing classification is derived from the same source and is based solely on the speed of ice formation:

  • Trace: Ice becomes perceptible. Rate of accumulation of ice is slightly greater than the rate of loss due to sublimation.
  • Light: The rate of accumulation may create a problem for flight in this environment for one hour. Unless encountered for one hour or more, de-icing/anti-icing equipment and/or heading or altitude change not required.
  • Moderate: The rate of accumulation is such that even short encounters become potentially hazardous. De-icing/anti-icing required to remove/prevent accumulation or heading or attitude change required.
  • Severe: The rate of accumulation is such that de-icing/ anti-icing equipment fails to reduce or control the hazard. De-icing/anti-icing required, immediate heading or altitude change required.


Effects

In-flight icing could lead to many problems, quite different in nature, such as:

  • Reduced lift – Increased drag
  • Uncommanded and uncontrolled roll – roll upset
  • Higher stall speed at lower angles of attack
  • Structural damage due induced vibration

Anticipated Impact on Crew

The stress level for the crew on a flight with icing conditions is significantly higher than during a routine one. The pilots should maintain high situational awareness and closely monitor ice formation as immediate diversion could be necessary.

The procedures for avoiding icing conditions include autopilot disengagement, change in altitude/heading or both and search for areas clear of clouds or with warmer temperature. All these pose significant amount of workload to the crew.

Once the first signs of flow disruptions are shown, the crew will put all efforts and attention to regain control. Therefore, during this phase an ATCO could expect broken, incoherent communication with the aircraft.

Suggested Controller’s Actions

Best practice embedded in the ASSIST principle could be followed (A – Acknowledge; S – Separate, S - Silence; I – Inform, S – Support, T – Time):

  • Acknowledge emergency on RTF
  • Take all necessary action to safeguard all aircraft concerned
  • May be required to suggest a heading
  • May be required to state the minimum safe altitude
  • Provide separation or issue essential traffic information, as appropriate
  • Emergency broadcast if necessary
  • After descent/climb request intentions

Expect

  • Immediate change of level and/or heading
  • Limitation in rate of climb/descent
  • Higher speed

Remember

In icing conditions:

  • Avoid holding - or provide holding flight levels/altitudes away from the 0°C isotherm
  • Enable continuous climb after departure – plan ahead – correct coordination between Twr/App/Control will ensure unrestricted climb
  • Keep safety strip clear – due to higher stall speed, aircraft experiencing severe/moderate icing would certainly keep higher approach speed. Tower controllers should keep runway safety strips clear during such landing. For details consult Runway Excursion article
  • AIREP to other ACFT, other units and MET – ATCOs should relay all pilot’s reports for adverse MET conditions to other aircraft concerned and to the meteorological office. Often even the less time-consuming ‘resume’ on the operating frequency provide valuable information for the pilots.

In icing conditions it may be appropriate to remind trainees or less experienced pilots of turboprop aircraft to:

  • Check anti-icing and de-icing systems
  • Pitot heating
  • Stall warner heating
  • Carburettor heating
  • Propeller heating / de-icing
  • Wing anti-ice / de-ice
  • Alternate air supply
  • Windshield heating
  • Descent with higher power setting to increase bleed air supply
  • Higher approach/landing speed due to increase of stalling speed

Defences

  • Pre-flight information - Crews should be supplied with latest weather information – they should also take the meteo-conditions into consideration while preparing the flight.
  • Anti-icing, De-icing - Anti-icing and de-icing procedures should be performed with full compliance to the requirements. The tower controller should take into account the hold-over time that is reported by the crew.
  • Personal awareness - ATCOs should stay alert for any extraordinary climb/descent. They should be ready to provide climb/descent to affected traffic and allow space for horizontal maneuvers. The aircraft, experiencing severe icing would certainly need increased separation as its flying characteristics would be degraded.
  • Information dissemination - The controller should pass all reports for adverse icing conditions to the incoming traffic. He/she should also pass this information to the meteorological office for further processing and dissemination.

Understanding the Stall-recovery Procedure for Turboprop Airplanes in Icing Conditions
Current pilot training typically emphasizes powering through a stall recovery with no loss of altitude. Nevertheless, when flying a turboprop airplane that has accumulated ice, lowering the
nose to reduce angle-of-attack is imperative. Here’s why.

Flight Safety Digest

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