Instrument meteorological conditions (imc)

• Communication failure occurs. ACFT sets transponder to A7600.

• Maintain last assigned speed and level for a period of 7 minutes, OR, if the minimum flight altitude is higher than the last assigned level, maintain the minimum flight altitude for a period of 7 minutes.

• Thereafter, adjust level and speed in accordance with the filed flight plan.

• If being radar vectored, or proceeding offset according to RNAV without a specified limit, proceed in the most direct manner possible to rejoin the current flight plan route no later than the next significant point, taking into consideration the applicable min. flight altitude.

Instrument meteorological conditions (imc)

Arriving aircraft with communication failure

The ACFT will proceed according to the current flight plan to the designated navigational aid serving the destination aerodrome and hold until commencement of descent.

Commence descent at, or as close as possible to, the EAT last received and acknowledged.

OR

If no EAT received and acknowledged, commence descent at, or as close as possible to, the ETA resulting from the CPL.

Complete a normal instrument approach procedure as specified for the designated navigation aid.

Land, if possible, within 30 minutes after the ETA specified,

OR

Within 30 minutes of the last acknowledged expected approach time, whichever is later.

ATC

In the event of communication failure, remember:

• Following the ASSIST code of practice

A ‘Acknowledge’ – S ‘Separate’ – S ‘Silence’ – I ‘Inform’ – S ‘Support’ – T ‘Time’

• Inform your Supervisor

• As soon as the failure becomes known, maintain separation between the affected ACFT and other ACFT based on the assumption that the ACFT will operate in accordance with the (ICAO) rules define for VMC or for IMC

• Recognize that this is an unusual situation which could become an emergency

• Establish whether the aircraft has:

• functioning transmitter

• functioning receiver

• neither

• if blocked frequency, establish the source:

• ground

• airborne

• inform all units concerned

• try to make contact with the aircraft on other available frequencies – 121.5 Mhz; 243.0 MHz; company frequency, or by any other available means, eg SELCAL; VOR: mobile phone etc.

• if control has to be transferred to another ATC station, radar hand-off procedure should be followed

CREW

IV. Pressurisation Problems

A failure of the cabin pressurisation system causes an immediate danger to the aircraft, crew and passengers. This is an emergency situation.

During high level flight, the aircraft cabin is pressurised in order to provide air for breathing and a comfortable environment for the human body.

Engine bleed air is conditioned (cooled down and mixed with outside air) and guided into the cabin. The amount and pressure of the conditioned air is regulated by a computer and outflow valves.

Pressurisation problems may be caused by:

• malfunction of the pressure outflow

• a malfunction of the pressure regulating valves

• a physical leak in the system

• a damage to a door or window

A rapid decompression in the cabin leads to loss of oxygen and increased gas pressure in the human body.

The drop in pressure will result in a sudden temperature drop, fog and reduced visibility in the cabin.

The difference in air pressure between inside and outside of the cabin leads to a wake effect where the wind sucks items towards the hole.

Loose objects will fly through the cabin and may harm passengers and crew.

Urns may boil over and bottles burst.

When the air from the cabin is lost, the time of useful consciousness (TUC) is short. This is the period of time during which a person can continue to function “normally”, without oxygen, before “blackout” occurs.

The time of useful consciousness can vary from 4 to 30 seconds depending on

• the altitude

• the size of the leak

• the size of the fuselage

It is vital that oxygen masks are deployed for pilot, crew and passengers during the TUC. The crew immediately puts on oxygen masks and then assists passengers to do the same.

The pilot can no longer use the standard headset for RTF communication. The pilot’s mask is therefore equipped with a microphone , and the earpiece is automatically switched over to a cabin loudspeaker.

The human body needs about 20 litres of oxygen per hour.

With increasing altitude, the density of air decreases and therefore the oxygen content decreases. The effect of this on the human body depends on the amount of oxygen available.

Medical Science has defined thresholds of altitude at which the human body will start to experience particular symptoms.

These are defined as bands of altitude, since tolerance will vary from person to person.

Reaction threshold (6000-9000ft)

Symptoms of oxygen deficiency start to show at this level, in the form of tiredness or exhaustion.

In the cabin of an aircraft the oxygen content is kept equivalent to a level of 6000ft or less, to prevent even those with the lowest tolerance from experiencing symptoms.

Disturbance Threshold (12000-15000ft)

At this level the symptoms are more serious. The lack of oxygen now also starts to affect our cognitive processes.

We start to lose the ability to think clearly and to make sound judgements.

We overestimate our abilities.

Critical Threshold (18000-24000ft)

At this level the density of air and oxygen content are only half of the value on ground.

More severe symptoms of oxygen starvation now also start to show.

Co-ordination of muscles is affected and the body will become paralysed.

Humans must leave this level as soon as possible.

In the even of depressurisation, the ACFT may stop its climb, request immediate descent or descend without warning.

If the problem cannot be resolved the crew will perform a rapid descent to below 10,000ft where passengers can breath unaided. The higher the ACFT the faster the rate of descent.

By doing this, the ACFT will descend through other levels and separation from other ACFT may be infringed.

These manoeuvres may cause injuries to passenger or crew.

ATC