CONTINUOUS IGNITION SELECTION

IN ADVERSE WEATHER

By Michel Palomeque

 

The recommendation for the selection of the continuous ignition selection in turbulence conditions has been deleted from the FCOM.

This paper explains the reasons of this deletion.

 

1. BACKGROUND DESCRIPTION

One A340 operator experienced double air conditioning pack closure in cruise.

Cabin altitude started to rise and "CAB PR EXCESS CAB ALT" ECAM warning was triggered resulting in an emergency descent.

The investigations have determined that this event was due to a combination of two failures:

. Internal FADEC failure leading to the triggering of " ENG MINOR FAULT".

. Engine bleed fault leading to the opening of the cross-bleed valve.

Indeed, both packs close if the following conditions are met simultaneously:

. The master lever is ON,

. The rotary selector is in "IGN/START" position,

. The engine is not running,

. The cross-bleed valve is open.

In the reported event, the first condition (master lever ON) was obviously fulfilled, the second condition (rotary selector in "IGN/START" position) was fulfilled because of the selection of the continuous ignition in severe turbulence.

The "engine not running" signal was also fulfilled because of the FADEC failure indicated by the memo "ENG MINOR FAULT".

The engine bleed valve was open because of the engine bleed fault.

The "engine not running" signal is done through a dedicated relay, which is powered by the aircraft and controlled by the FADEC.

In the light of our experience on the AIRBUS aircraft, it appeared that some troubles were already reported concerning this signal.

At the opportunity of this event, AIRBUS decided a complete review of the use of the continuous ignition

An aircraft wiring modification was at once defined to reinforce the validity of the "engine not running" signal.

In between, in order to cover the aircraft without this modification, (even if the probability to get a double failure on the aircraft is quite low), an OEB was issued both for A330 and A340.

This OEB recommends in particular to not select the continuous ignition in cruise in severe turbulence.

The reasons for this recommendation are explained below.

Note: An electrical wiring modification has also been defined to solve this problem: the OEB is cancelled when this modification is embodied.

2. INSTRUCTION AND LOGIC DESCRIPTION FOR EACH ENGINE

Herebelow are listed, for each engine manufacturer, the instruction to be applied in case of adverse weather as far the continuous ignition is concerned.

2.1. CFM Engines

2.1.1. Specific Operating Instruction

The ignition system is designed for continuous use. However, continuous ignition is required only for those phases of operation determined by the aircraft manufacturer and specified in the aircraft operation documents.

Normal engine operation does not require the use of continuous ignition.

Ignition system component life can be prolonged with judicious use of ignition.

In icing conditions, in flight, if fan icing is suspected, activate continuous ignition.

Ignition is also automatically selected by FADEC for some flight conditions such as nacelle anti-ice selection.

CFM56-5A: if during descent, moderate to heavy precipitation (rain, hail, or sleet) is encountered or anticipated, select engine cowl anti-icing on. When engine cowl anti icing is switched on, the FADEC automatically controls continuous ignition and selects a higher idle schedule.

This is cancelled with the embodiment of several modifications (igniters are replaced, modified cone…).

When these modifications are installed, no special procedures are required to maintain safe operation.

CFM56-5B/5C: if during descent, moderate to heavy precipitation (rain, hail, or sleet) is encountered or anticipated, no special procedures are required to maintain safe operation.

2.1.2. FADEC logic

The ECU has the capability to detect an engine flame out. When a flame out condition is detected, continuous ignition is automatically selected.

A flame out is detected when a rapid decrease in N2 occurs, or N2 is less than idle RPM (i.e. less than around 57%).

If the rotary selector is in NORM position and a successful re-light is achieved, continuous ignition is discontinued after 30 seconds.

If the engine does not re-light, continuous ignition stops when N2 is below 40%.

This automatic ignition feature aids in the re-light of the engine in case of flame out or if the master lever is turned OFF inadvertently then back again.

During ground auto-start, the FADEC selects one igniter only.

A second igniter might be turned ON for a delayed or no light condition.

During a manual start or in-flight auto-start, the FADEC selects both igniters ON simultaneously.

2.2. IAE engines

2.2.1. Specific Operating Instruction

The V2500 ignition system is capable of continuous operation; however, the igniters should be turned OFF to conserve the life of the ignition system components whenever ignition is not needed on the ground or in flight.

Continuous dual ignition should be manually selected when operating in moderate to severe turbulence or operation in heavy rain.

This selection offers maximum flameout protection and restart capability in the event of an engine thrust loss.

2.2.2. FADEC logic

Each channel of the FADEC is interfaced with two exciters. Each exciter provides one ignition level. For each auto-start on ground, the FADEC selects alternately exciter 1 or exciter 2. For each air start or manual starts, both exciters are selected by the FADEC.

Note: If selector is set to NORM position after engine 1 starting and then reset to IGN START prior to starting engine 2, the result is the selection of continuous ignition on engine 1.

In the air, continuous ignition is selected whenever the rotary selector is in IGN START (the rotary selector does not need to be cycled).

Continuous dual ignition is automatically selected when any of the following conditions exists:

. Engine cowl anti-icing is selected.

. Maximum takeoff thrust is selected (EPR or rated N1)

. Flexible takeoff thrust is selected (EPR mode)

. Approach idle logic is selected by the EIU discrete

. EIU data has failed (ignition and/ or bleed configuration data);

. In-flight unscheduled sub-idle or surge occurs (flame out).

 

Notes:

1. Flame out conditions exists until engine N2 goes above around idle RPM (i.e: less than around 51%).

2. Upon detection of engine stall, the ignition is turned on and a timer is initiated.
This timer functions as a method of returning the engine back to normal operation.

The igniters will remain on for an additional 30 seconds after the timer has timed out.

3. Upon master lever ON, after being turned off, ignition will be maintained for 30 seconds provided the engine is above idle.

 

2.3. GE engines

 

2.3.1. Specific Operating Instruction

The ignition system is designed for unlimited use for starting, takeoff, landings and adverse weather conditions.

The ECU has a logic which detects a flameout and automatically energizes dual ignition when a flameout is sensed.

If ignition has not been manually selected by the crew (rotary selector NORM) and a re-light does not occur before N2 is below 10%, then ignition is discontinued and a complete air start sequence must be initiated.

2.3.2. FADEC logic

The ECU selects continuous ignition (both igniters ON) when commanded by the rotary selector or when the nacelle anti-ice is active.

The ECU has the capability to detect an engine flame out. When an engine flame out is detected, continuous ignition is selected.

A flame out is detected when a rapid decrease in N2 occurs, or N2 is less than idle RPM.

If the rotary selector is in NORM position and a successful automatic re-light is achieved, continuous ignition is discontinued after 30 seconds.

 

2.4. PW engines

2.4.1. Specific Operating Instruction

The EEC controls both igniters (via relays) individually during ground auto-starts and simultaneously during manual and in-flight starts. Both igniters are selected for auto-starts attempts in-flight and for all manual starts (in-flight and on the ground).

Selection of continuous ignition (both igniters ON) is available to the flight crew by setting the mode selector to the IGN/STAR position.

The automatic selection of continuous dual ignition is provided by the EEC during specific or adverse conditions.

The PW4000 ignition system is capable of continuous operation; however, the igniters should be turned OFF to conserve the life of the ignition system components whenever ignition is not needed on the ground or in flight.

Continuous dual ignition should be manually selected when operating in moderate to severe turbulence, operation in heavy rain or volcanic ash.

When operating in or near moderate to heavy rain/hail, select IGN START for continuous ignition.

This selection offers maximum flameout protection and restart capability in the event of an engine thrust loss.

2.4.2. FADEC logic

The control of the ignition exciters is accomplished automatically by the EEC during starting.

Both igniters are energized during in-flight start attempts.

During a manual start procedure, both igniters are energized when the master lever is moved to the ON position.

CONTINUOUS IGNITION is selected automatically when any of the following conditions exists:

. Engine cowl anti-icing is selected.

. Maximum takeoff thrust is selected (EPR or rated N1)

. Flexible takeoff thrust is selected (EPR mode)

. In-flight flameout is detected

. Approach idle logic is selected

. In-flight surge is detected.

Note: A flame out is detected when N2 is less than idle RPM.

2.5. RR engines

2.5.1. Specific Operating Instruction

As part of the ignition control features, the FADEC system provides an auto re-light function. On detecting a flame out, the EEC will command continuous ignition to recover the engine.

In the event of a flame out, the auto re-light function commands the igniters ON and try to re-light the engines. If N3 drops below 35%, auto re-light has failed to re-light.

Note: there is no specific recommendation for the selection of continuous ignition in heavy rain, hail or volcanic dust.

 

 

2.5.2. FADEC logic

Auto re-light provides the facility to minimize the period of igniter system energisation whilst providing flame out recovery protection equivalent to continuous ignition.

The FADEC is capable of detecting the occurrence of an engine flame out by monitoring combustor pressure and rate of change of HP shaft speed.

In the event of FADEC detecting an engine flame out condition, the EEC energizes both igniters while the condition exists and for 10 seconds after the condition is cleared.

Engine starting following a flame out is never automatically initiated by the EEC but will be commanded by the pilots through the current procedure.

The following igniter selection is made:

. One igniter

Use of igniters A and B will be alternated during consecutive automatic ground starts.

. Both igniters

Auto re-light, engine in-flight restart and continuous ignition.

The continuous ignition is selected automatically by the EEC following inadvertent cycling of the master lever with the engine running. The EEC assumes this occurrence should the engine HP shaft speed be above 50% and the master lever returned to the ON position from the OFF position. Continuous ignition is selected for a period of 10 seconds.

In addition to the auto start logic, a quick re-light function is available to provide an immediate re-light capability.

If the pilot inadvertently switches the master lever to OFF or the engine runs down, the pilot can invoke the quick re-light logic by returning the master lever to the ON position (or by cycling it to OFF then ON). Providing the master lever is returned to the ON position within 30 seconds and N3 remains above 10%, then the HPSOV is opened and both igniters energize immediately.

The above logic is operated independently of the auto start function, regardless of the rotary selector position and provided the above conditions are met, it takes priority over the auto start function.

In conclusion, the behavior of the different engines can be summarized as follows:

An "X" in the table indicates the automatic selection of the continuous ignition.

 

	CFM56	IAE	GE	PW	RR
Nacelle anti ice selected	X	X	X	X
Max/flex takeoff selected		X		X
Approach idle selected		X		X
In flight surge detected		X		X
In flight engine flame out detected	X	X	X	X	X
Inadvertent cycling of master lever	X	X	X	X	X

 

In the same manner, the engine manufacturer recommendation can be summarized as follows:

An "X" in the table indicates the recommended manual selection of the continuous ignition.

	CFM56-5A	CFM56-5B/5C	IAE	GE	PW	RR
Heavy rain at takeoff/ in flight			X		X
Moderate turbulence in flight			X		X
severe turbulence in flight			X		X
Moderate precipitation in descent	X (without mod)		X		X
heavy precipitation in descent	X		X		X

 

 

3. THE AUTO RELIGHT FUNCTION

3.1. The auto-relight description

The auto relight is a system designed to detect and recover the engine from flame out at any engine speed by energizing both igniters.

 

 

 

 

The system consists of the ECU (EEC), N2 (or N3) speed signal (depending on the engine) and for the ROLLS ROYCE engine, of a compressor delivery temperature signal (T30).

. The auto relight function is an integral part of the ECU (EEC)

. ECU (EEC), N2 (N3) signal or combustor temperature signal failure is a NO DISPATCH.

The ECU (EEC) auto relight function assumes engine flame out and energizes both igniters if:

. The rate of N2 (N3) deceleration exceeds a certain threshold. (CFM, GE, RR) or

. N2 (N3) speed falls below idle. (All engines) or

. Severe rain or hail ingestion is detected: drop in the compressor delivery temperature T30 (RR).

The auto relight flame out detection process is completed at the rate of the computer clock , which means more rapidly than two consecutive sparks.

Some tests have been conducted on some engines.

It is proven that auto re-light provides faster flame out protection than continuous ignition selected by the crew: the tests have shown that auto re-light detected a flame out and energized igniters in less time than it took for two consecutive sparks of continuous ignition system.

Thereby, the auto-relight function is at least as efficient as the continuous ignition.

3.2. Auto-relight in service experience

In service experience has shown that the continuous ignition was selected automatically:

When in flight unscheduled sub-idle thrust occurred on different engines/ different aircraft/for different reasons, continuous ignition was every time selected automatically by the FADEC.

3.3. ROLLS ROYCE flame out testing

Intensive tests have been conducted on the Rolls Royce engine to check the efficiency of the auto relight function.

. Hail ingestion testing: simulation of critical descent through 10 g/m3 of hail at 15000ft, at maximum allowed aircraft speed.

. Rain ingestion testing: Max takeoff test: water ingested for 3 minutes, water flow 90% in excess of requirements.

. Idle test is equivalent to core liquid water flow 350% in excess of worst rain ingestion case.

. Simulation of critical descent through 20g/m3 of rain at 20000ft.

. Combustor stability testing at sea level and altitude

. Wet re-light testing at altitude.

During all these tests, thanks to the auto-relight function, no flame out occurred.

4. IGNITION SYSTEM DRAWBACK

4.1. The quench pressure

Most engine manufacturers use air gap igniters, which quench (do not spark) at high combustor pressures: the threshold is basically around 300 PSI.

The implication is that continuous ignition is inoperative at high engine powers.

The quench pressure reduces with igniter deterioration.

When the quench pressure reduces, the igniters spark very often when continuous ignition is selected.

Because they spark very often, they are deteriorated more rapidly.

Because they are deteriorated, the quench pressure reduces…

 

Note: The TRENT 800 uses surface discharge igniters, which energize under any operating conditions:

. Igniters are designed to operate at high temperature and at high combustor pressures: they still operate at 600 PSI, which corresponds to high engine power.

. Igniters currently utilize advanced material technology.

. System has not been designed for continuous ignition. Continuous ignition selection compromises igniter life.

4.2. The corona effect

On some engines, early failures on ignition leads are the leading cause of engine-related delays and cancellation (predominant lead failure mode is flashover across connection seals).

The problem is initiated by a corona discharge within the mated material.

The observations and bench test results have confirmed the presence of corona: corona is a high voltage electrical phenomenon that causes ionization of surrounding air and low level electrical discharge.

The corona can cause degradation of insulation and carbon deposits.

The carbon allows higher level electrical discharge by conduction and thus allows eventual flashover at seals.

The use of continuous ignition increases the duration of the corona effect: this allows application of ignition exciter voltage at engine/flight conditions where igniters can not fire.

 

 

 

5. REASONS FOR THE SUPPRESSION OF THE CONTINUOUS IGNITION SELECTION RECOMMENDATION IN ADVERSE WEATHER CONDITIONS

 

1/ As recommended by all the engine manufacturers, the igniters should be turned off to conserve the life of the ignition components whenever ignition is not needed in flight or on the ground.

In any case, the ignition system must be capable to restart an engine when a flame out occurs: this is obviously safety related.

To achieve that, it is better to use the igniters only when necessary.

If an aircraft is dispatched with the igniters already deteriorated, in particular for a long-range flight, the probability to get the igniters inoperative after 10 hours of intensive use during cruise is not negligible.

The igniter life can be extended with judicious use of ignition.

2/ the use of continuous ignition has been identified as a contributor to the formation of corona: corona causes ignition lead failures (flashover across the silicone seals).

3/ the efficiency of the auto re-light has already been tested on some engines.

4/ In-service experience has shown that the continuous ignition was automatically selected every time the engine was at sub idle.

This was noted in particular on the A320 engines.

5/ the selection of the continuous ignition does not preserve the engine to shut down but allows only the engine to restart as soon as possible.

6/ the probability to get a dual pack loss in cruise when the continuous ignition is selected is higher than the probability to get it in the other flight phases.

Taking into account all the above points, considering also the fact that the takeoff and the approach phases are considered as critical (human factors):

It has been decided to suppress the recommendation to select the continuous ignition in severe turbulence conditions in cruise.

 

6. FCOM RECOMMENDATION SUMMARY

The Standard Operating Procedure (S.O.P) recommendations for the continuous ignition selection are listed below:

FCOM 3.03.11 p1

BEFORE TAKEOFF

.
– ENG START selector………………………………………….AS RQRD

Select IGN/START if:

Depending on the engines, it is written either:

a) CFM and GE engines:

. The runway has standing water for the engines which do not select automatically the continuous ignition when max TO thrust or FLEX thrust is selected.

. Heavy rain is falling

. Heavy rain or severe turbulence is expected after takeoff

b) RR and PW engines:

Heavy rain or moderate or severe turbulence expected after takeoff

c) IAE engines

Heavy rain or severe turbulence is expected after takeoff

And

Note: Continuous ignition is selected automatically if the ENG ANTI ICE pushbutton switch is ON: this is true for all the engines except for the ROLLS ROYCE engines.

.

FCOM 3.03.13 p 1

AFTER TAKEOFF

– ENG START selector………………………………………….AS RQRD

Select IGN/START if severe turbulence or heavy rain is encountered.

FCOM 3.03.18 p1

ILS APPROACH

INITIAL APPROACH

- ENG START selector……………………………………………AS RQRD

Select IGN/START if the runway is covered with standing water, or if heavy rain or severe turbulence is expected during approach or go-around.

FCOM 3.03.19 p1

NON PRECISION APPR

INITIAL APPROACH

- ENG START selector……………………………………………AS RQRD

Select IGN/START if the runway is covered with standing water, or if heavy rain or severe turbulence is expected during approach or go-around.

FCOM 3.04.91 p2

ADVERSE WEATHER

SEVERE TURBULENCE

ENGINE

Set the ENG START selector to IGN/START.

 

This recommendation is no longer requested, since the auto relight function is considered efficient in cruise conditions.