Emergency procedures

General

Emergencies and abnormal procedures arise from two sources: from the pilots or the ATC side, depending where they occur. If a pilot gets into trouble then he declares an emergency. Most likely an emergency is affecting your surrounding ATC stations too. Therefore you have to coordinate with them to safely manage the situation.

ATC emergencies

What if the controller discovers something which will require immediate reaction (like deer on the runway, a broken plane unable to vacate, ...)? There is no mayday call for ATC controller - but you have the power to react: use the available urgency phrases, give a reason for the emergency and everything should be safe again.

Usually, such an emergency needs more than one message. Deploy them in the order of urgency: First the closest affected aircraft, second all others (other ATC, other pilots). Bear in mind that you want to make sure that the closest aircraft needs to be out of danger. See if the aircraft reacts and wait for the readback, if the time allows it.

APP: AUA186T, collision alert, climb FL100, say again climb FL100, expedite!
A/C: climbing max rate, traffic on TCAS, AUA186T.

TWR: DLH56H, go-around, say again go-around, runway closed
A/C: going around, DLH56H.
TWR: All stations on final: runway 16 is closed, prepare for go-around on my command.
TWR -> APP (on TS): Approach, runway 16 is closed, all aircraft's are going around. Wait for further instructions.

Abnormal Procedures

Abnormal procedures are only triggered by pilots. There are only three possible categories:

• pre-warning ("we have a problem"),
• Pan-Pan,
• Mayday.

As stated the section before, an "ATC emergency" as such is not an abnormal procedure.

1. Houston, we have a problem

It is quite common that the emergency call does not come right away. The crew might discover something (an indicator in the cockpit) and follows the letters: DODAR (Diagnose, Options available, Decide, Allocate task, Review). In this case, a short message to ATC is common, that something is wrong:

NLY2863: Tower, Flyniki 2863, we have a problem, standby.
TWR: NLY2863, standing by.

If you are a controller and hear something like this, prepare yourself! Oversee the situation, you might wait with the next takeoff clearance, think of scenarios like: where could the aircraft turn back? Which diversion can it reach? The pilot will call you back in a few seconds with precise information.

2. Pan-pan

Pan-Pan is an urgency announcement of the crew, which is used if an aircraft is endangered but not actually at risk. There is no need for immediate action, but the flight-crew calls for a preferential treatment. The crew can continue flying under normal or restricted conditions and usually land regularly.

SWR111:   Swissair one-eleven heavy is declaring pan-pan-pan. We have hydraulic warnings, requesting return to a convenient place, I guess Boston.
CZQM_CTR: Swissair one-eleven heavy, pan-pan received, turn left direct Boston VOR, descend and maintain flight level 100.

---

Radio communication Failure (RCF)

If RCF occurs, you as controller might notice it first, because the aircraft does not react. If the pilot notices RCF, he/she will squawk 7600 (das "6" reimt sich auf "hör' nix"), and the letters "RDOF" show up in the Euroscope tag. What should you do?

• First, see if any immediate action is imminent, like: clearing a conflict by diverting other aircraft.

• Second, determine what RCF it is: does the pilot hear or is his aircraft completely deaf? For this purpose, you ask the following:

Controller: Leipzig 123, received radio communication failure. If you read, squawk Ident.

If an ident signal is received, then things are easier: The pilot can receive calls normally. You might want to direct the pilot with as little commands as necessary - maybe the radio receiver will fail too. If traffic permits, clear him/her to final.

If the pilot sends no ident signal, then you (and even more important: the pilot...) need to know the RCF failure procedures. The airplane will follow this procedure and land. Controller's job is...

• to get other pilots out of the way
• to inform other ATC "upstream" about the situation (put a message into the text tag)
• to monitor the aircraft: A RCF failure could turn into an emergency (you don't know, what's all wrong in the plane), and the pilot might not strictly follow the RCF procedure. Remember that the pilot needs the radio for local QNH, which might have changed since departure (local QNH at destination is part of the briefing).
• to bear in mind that the runway config might have changed and the pilot still approaches the "old" config.

A RDF procedure typically follows one rule: Pilot follows orders as far as he has received them, and follows standard procedures thereafter.

1. If the pilot has received the airport info (ATIS or other), then it will follow it (QNH, runway in use). Caution: The only way ATC can know this is if there has been contact, for example with APP, where the pilot has reported "ATIS on board". If in doubt: see what he is doing and figure out what he knows.
2. If the pilot has not received it, then it will follow standard runway configuration according to the RDF rules for the approached airport, if there are any.
3. Typically, procedures include flying standard STAR and transition and profile in the charts. Some airports include holdings for a predetermined time.

See the RCF procedures for [LOWW]. There are no precise rules for the other Austrian airports.

• In real life, pilots take their cellphone and call in (emergency phone number is on the charts - DON'T CALL THEM if you find them on VATSIM!). VFR pilots are expected to follow clearance received, or if they don't have clearance, to divert to an uncontrolled airfield.
• At VATSIM, expect pilots to follow the route they have been cleared or (if not cleared) filed.

---

Go Around and missed approach

A Go-Around can be seen as abnormal procedure. Two different procedures can be followed:

1. Missed Approach published in the Charts. APP should be informed about it (via teamspeak or PM).
2. Individual handling like heading and altitude. This has to be coordinated with APP.

Remember: If necessary for safety, TWR can (and must) tell the aircraft virtually anything the aircraft is able to. If the reason for go-around is in the direction of the go-around procedure, then "proceed as published" does not really make sense. TWR might tell the aircraft to turn off left or right immediately.

Phraseology differs according who announces it:

• If ATC issues go-around, then the phraseology expresses urgency. Listen to the readback - it is vital that the pilot received it. In this example, the go-around is individual:

TWR: Leipzig 123, go-around, say again go-around, turn left immediately direct SNU, climb 5000ft, acknowledge.
LHA123:LHA123 is going around, turning left SNU, climbing 5000ft.

• If the pilot goes around, then he/she announces it. In this example, standard go-around pattern is used:

LHA123: Lepzig 123 goes around.
TWR: Roger, Leipzig 123, go around as published

As a courtesy, you could tell the pilot the go-around procedure anyway - many of them don't know it. Caution: Vienna (due to crossing runways) has situations, where go-around procedures conflict with departing aircraft. Watch out.

---

Rejected take-off

Rejected take-off procedures can quickly turn into a nasty situation, if ATC does not react immediately and correctly.

If the pilot rejects take-off

The pilot's phrase is:

LHA123: LHA123 rejecting takeoff.

For ATC this means, that the approaching traffic should immediately receive go-around instructions and the APP should be informed about it. Do not expect the aircraft to leave the runway immediately - something serious might be wrong, so be prepared to change the runway (if you have, like in LOWW).

If TWR withdraws take-off clearance

The situation is even worse, as the aircraft might be rolling already and be beyond v1 where it can't stop (and you as TWR don't know the specific V1 speed). In this case, TWR has to do the following:

If the aircraft has commenced take-off:

"TWR: LHA123, Stop immediately. I say again (Callsign) stop immediately - acknowledge".

If the aircraft is already past v1, then the pilot will respond, and you will have to tell second-best solutions (if you don't have a solution, then tell the problem - the pilot has to find the solution):

LHA123: unable, LHA123
TWR: LHA123, aircraft on the runway, intersection 2.

If the aircraft has not commenced take-off:

"(Callsign) hold position. Cancel take-off - I say again, cancel take-off - acknowledge".

• If you have arriving traffic you must assume go-around instructions until the runway is free for the next approaching aircraft.
  

---

Malfunctions of airport navigation equipment

Strictly spoken, malfunctions of airport equipment is not an abnormal procedure. Every airport has procedures for failing equipment. If ILS fails, then there are VOR approaches, or NDB or visual or RNAV approaches. If runway lighting is short-circuited, then you just tell it or close the runway (if it's dark or foggy).

Anyway, Malfunctions of airport equipment (broken ILS, VOR or NDB) cannot be simulated at VATSIM (navaids are part of pilot software). You may simulate failing airport equipment and issue different approaches (like NDB approach, if you simulate failing ILS), but it is not in your hands, if pilots follow.

It could happen that a pilot reports a malfunction (in case his software simulates such a failure - but this failure only affects him, and you could treat it as airport failure: if the ILS approach is dead, give a VOR approach.

And Still, you could simulate airport malfunctions for all pilots for fun. In this case, you should know the other approaches available, offer alternatives (and communicate them between TWR and APP). If weather permits, you can offer visual approaches.

---

Priority Landing

This is an easy case: A fully functional and communicating aircraft has only one grievance: Get down as soon as possible. Most reasons are medical issues, live organs, low fuel (at too low fuel the pilot will report an emergency), or VIPs on board. Also government flights can request priority handling.

Handling is easy: Issue instructions (vectors, descent, and on ground: a gate close to arrival) that best meet his/her request.

---

Runway closure

This procedure is triggered by the TWR controller. There are two different cases:

• Immediate closure: If the runway has to be closed immediately (broken down aircraft, debris, deer, ...), then the runway closure is imminent:

1. approaching traffic receives go-around
2. APP receives request, not to hand off any more aircraft
3. GND receives information to halt taxi clearances to this runway
4. New runway is opened, GND and APP informed (or airport closed)
5. ATIS adapted.

• Step-by-step closure: If the reason for closure can be forseen (wind changes slowly, noise abatement rules), then the runway closure is more orderly:

1. APP receives request not to hand off any more aircraft and/or divert to new runway
2. GND receives request not to issue taxi clearances to this runway, and the new one
3. Remaining aircraft receive landing clearance
4. New runway will be opened
5. ATIS adapted
6. APP and GND are informed.

3. Mayday emergencies

There is one rule of thumb: A-S-S-I-S-T:

• A - Acknowledge: Make sure you understood the nature of emergency and acknowledge accordingly
• S - Separate: Don't forget to establish/maintain separation
• S - Silence: Impose silence on your frequency if necessary. Don't disturb urgent cockpit actions by unnecessary transmissions.
• I - Inform: Inform other sectors or units.
• S - Support: Give maximum support to pilot and crew
• T - Time: Allow pilots sufficient time to work on their problems.

4 phases which govern it all

If a pilot discovers a condition of being threatened by serious and/or imminent danger and of requiring immediate assistance, then he/she will issue an emergency call.

A basic principle is important: The pilot is in command, ATC is here to assist. Don't tell the pilot how to fly the aircraft - it's not your job, and you are not there. The pilot says his intentions, and ATC reacts. This does not mean that you don't order the pilot to do things, but they should be according to his/her intentions, not yours.

Mayday procedure has the following phases:

1) Mayday call by the pilot:

LHA123:   Mayday, mayday, mayday. AUA123 engine failure. Descending below FL240, Request vectors to nearest suitable airfield.
LOWW_APP: AUA123, emergency received, engine failure. Next airfield is Vienna Airport, turn left 020°, 60 miles.

Pilot calls "mayday" three times, the nature of his/her emergency, what his/her intentions are and what he/she needs now. ATC reads back the emergency.

• The emergency squawk code is 7700, and setting it enables all controllers to see the problem (in red, on their screens). Don't advise the pilot to set squawk 7700 at this time, the current problem on bord is bigger than a wrong squawk code. Pilots should do it on their own, and if they don't, you can tell them later, when the situation is more stable ("if able, squawk 7700").

There may be situations, where the pilots has no time to ask for anything (yet): He just declares the emergency. Then your job is to stand by and oversee the situation (available runways to turn back, available altitudes and turnings, and to inform other ATC).

LHA123: Mayday mayday mayday, LHA123.
LOWW_APP: Leipzig 123, standing by.

Squawk and callsign-change?

Two things can be seen in youtube videos and around: The mayday Squawk 7700 ("seven" goes well with "going to heaven"), and that "mayday" is amended to the callsign. LHA123 turns to be "mayday-leipzig-air-123".

Contrary to this, there is no obligation to do that:

• Pilots might be too busy to set the squawk. It's on their checklist in the QRH, but not at the top. 7700 is good to inform other ATC up- and downstream (the tag turns violently red), but you know it, and you inform the others earlier than the pilots.

• To add "mayday" to the callsign makes the callsign quite long, and who needs it? The pilot knows he's in trouble. You know too. In IFR, others are not supposed to be bothered.

2) standard response by ATC

The pilot has a few seconds to do (avigate, navigate), and ATC too: He/she needs to inform other pilots and fellow ATC stations. In dramatic situations where you need as much frequency as possible, you can order radio silence:

LOWW_APP: All stations, all stations, emergency in progress. No calls, no readbacks.

With this call pilots know: They should continue as told (their SID, transitions, STAR). They only call ATC, if safety is at stake. If they receive ATC orders, they don't read back - they just do it.

Then ATC informs other controllers via intercom, Text or teamspeak, and collects the necessary information.

LOWW_APP->LOWW_CTR: mayday LHA123, engine failure, no handoffs
LOWW_APP->LOWW_TWR: mayday LHA123 engine failure, request rwys available
LOWW_TWR->LOWW_APP: all rwys available, wind 320°6kt

Generally, the controller makes sure that he/she does not get any traffic any more: no takeoffs from below, no handoffs from above.

Beyond this point, there is no fixed phraseology. ATC speaks with pilots to make sure that he is served best. With the information gathered, ATC will turn back to the pilot and figure out next steps.

LOWW_APP: Leipzig 123, Vienna reports all runways available, 52 miles to final, for runway 11 turn left 300°, wind 320°4kt. When able, report situation.
LHA123: Leipzig 123, left engine out, right engine 40%, descending 1800, 48 passengers and 5 crew on board, fuel for 1 hour.

ATC's information is important to pilots: Where can I land and how far it is? Pilots say, that accurate distance is vital, so use the distance tool on your scope.

The pilot's first information is important for controllers: What is wrong? That says a lot about the ability of the pilot to control the aircraft - but do stick to the rule that the pilot flies: Let him tell you what he needs. The last info is important for real life: Emergency services on the ground need to prepare for rescue.

3) All following measures

There is no strict rules beyond the standard response. Pilot and ATC communicate the way that suits best their situation. The following guidelines are vital:

• Pilot tells, ATC delivers. On the flight plan, you read "PIC - pilot in command" - let him stay in command, and let him decide, which command he wants you to take over.
• Communication: least possible, as precise as possible. During an emergency, the cockpit is a beehouse. The more time pilots have for themselves, the better. If a pilot does not answer: stay calm - the pilot might follow the rule 1-aviate 2-navigate 3-communicate and might not have reached step 3. He/she may say "ATC standby" or just nothing. To quote a real-life example: [don't do this.]
• Clear needed airspace: Noone needs to turn away, if he/she is not in the way. Some traffic might continue as normal. You might consider premature handoffs to other controllers or holdings. As a rule of thumb: don't produce more emergencies than you have already.
• Pilot has the choice of frequency: An aircraft can stay on the frequency (eg. TWR) through all stages of his/her emergency or change, if he/she wishes. ATC will adjust. Assume that you have an engine failure at FL100 approaching LOWW. The pilot is on the APP frequency and does not want to change. Either APP coordinates with TWR landing clearance and issues it, or TWR enters the APP frequency and issues landing clearance him/herself.

Consider the example LHA123 from above: He is off runway 34 in Vienna with the left engine dead. Pilots know that you are not supposed to turn onto a dead engine, so the original "turn left" for runway 11 is maybe a bad idea. But it is pilot's decision. If he wants to turn left, he should turn left for 11. Most likely, he will come back to you:

LHA123: unable for left turns, alternatives please.
LOWW_APP: LHA123, you can turn right 300° for runway 29, 24 miles to go, or turn 340° for runway 16, 36 miles to go.
LHA123: Going for 29, LHA123

4) End emergency procedure

At some stage, the emergency is over, and the aircraft is happily on the runway or unhappy in the wood. At this stage, all other pilots and ATC need to know that it is over:

LOWW_APP: All stations, emergency procedures terminated. All operations return to normal.

Then, cleanup starts: There might be a dozen aircraft in holdings to bring in, runways to clean and aircraft on ground to get out.

Emergencies - the cases

This list is open - be prepared to react to the unusual. If you discover an emergency which is not covered here, feel free to add it.

Loss of Cabin pressure

Loss of cabin pressure is an annoyance at FL150, but a real danger at FL340. Expect aircraft to do a rapid descent without authorisation to about FL100.

eport minimum safe level to the pilot as soon as you can. If he descends south of Innsbruck (MSA=FL140), he will crash at FL100. If you don't find MSA quickly enough, then take MRVA (on the Euroscope screen) and deduct 1000ft, that's better than nothing.

As controller, you have to clear the way, and you can expect that the pilot wants to land as soon as possible. If the pilot acts by the book, he/she will turn off course by about 30° (to avoid hitting aircraft below). Be prepared that the emergency situation escalates, as there is a reason for the loss of cabin pressure which could cause other systems to fail too.

Fuel dumping

Aircraft who need to return right after takeoff often have too much fuel to land, and they need to dump it.

1. Assign a Dumping area - avoid refineries, industrial plants, densely populated areas.
2. Assign a minimum altitude: No dumping below 6000ft.
3. CareSeparation - 10nm to eiter side; above 1000ft, below 3000ft, behind 15 minutes or 50nm (that's a lot!)
4. Alert other aircraft: General Call on frequency (in real life also on 121.5 and 124.4): "all stations, use caution, fuel dumping in progress over <position> from <FL> heading <...>, avoid flight within 10nm from dumping area".
5. The Pilot may impose radio silence until a mutual agreed time.

Loss of Hydraulic Pressure

This is a major problem, depending on the degree of failure: On large aircraft (except B777, which is merely electric - in turn, the batteries tend to burn), all systems for manoever depend on hydraulic pressure. First thing after the usual introduction is to ask the pilot about his/her status - which systems are affected and what this means. It could mean that the pilot cannot deploy speedbrakes (that's trivial) or flaps (that means a long runout on the runway), or cannot perform certain manoevers (like left or right turn etc, or that the plane reacts very slow). As ATC, you should offer the nearest suitable airport and increase separation. Also a loss of hydraulic pressure can escalate quickly into a worse emergency.

Generator failure

...means, that the aircraft's electric systems purely rely on the battery, as the battery charger is dead. The aircraft can continue as normal, but not for long - battery power will fade. Usually, a generator failure is a "pan"-case, which will escalate into a "mayday", if power is out. Bear in mind that radio communication drains battery, and that pilots will talk as least as possible. When battery power is gone, there is no squawk and no radio communication (unless the aircraft has a RAT). With generator failure, aircraft usually land priority.

Electrics failure

... means that an unknown range of systems in the aircraft which rely on electricity fail. Electrics failures could escalate quickly - short circuits could produce fire.

• Pilots will most likely want the closest suitable airfield.
• A major power consumer is radio communication. Consider offering solutions which need very short answers. If all electric systems fail, then you won't hear the mayday call - no power, no radio.
• Electric systems primarily power navigation and communication. Expect to guide a "blind" aircraft and consider directions for routing, and monitor headings.
• Expect abnormal approach procedures - shortened, steeper, but maybe with smaller bank angles - the pilot will tell what he is able to.

Smoke in Cockpit - Rauch in der Flugkanzel

Smoke in cockpit is a very challenging situation, as pilots can't see properly. The best way is not to lose overview in the situation. Time is critical - the aircraft has to come down really quickly. Contrary to other emergencies, smoke in cockpit means a "talk-down" approach. Supply the pilot with vectors and distance to the nearest airfield. The pilot might ask you to call out vital information (it could be that he/she cannot see his/her instruments)in regular intervals (several times per minute), like:

Mayday Leipzig 123, speed 220, altitude 6000ft descend, heading ok, runway 11 miles

Such an emergency should definitely land at the next suitable aerodrome, and maybe on a separate runway in case anything goes wrong. Increase separation considerably.

Engine Failure at takeoff - Triebwerksversagen beim Start

Engine failures are the most dangerous incidents that can happen: Altitude over ground is low, the aircraft is at full thrust and full of fuel, speed is low. This is the cocktail for a nasty situation. Most frequent reason is bird strike.

Most likely, the bird strike happened already on ground above v1, and the aircraft has to get into the air with reduced engine power. Therefore, TWR is most likely to receive the mayday call. High insecurity adds to this, as it may take time to assess the degree of damage. How much thrust is still left? Is climbing an option? Does the aircraft need to make an emergency U-turn or does it have enough time for a controlled approach?

Workload in the cockpit is very high. Quick, short response and coordination is the key to this situation. More important than in any other situation is to offer solutions, but let the pilot decide. The pilot will tell you, what he is able to and what his intentions are.

Be prepared that the aircraft may not follow the filed SID. It will most likely climb slower, step-climb and need a larger turn radius. In turns, the rate of descent might double.

Here is a perfect example, how it should work:

1. Mayday call: as precise as possible: Mayday, callsign, problem, intentions, request

Acft: Mayday mayday mayday Thomson 253H, engine failure, continuing northwesterly, inbound towards Wallasey.

2. Answer ATC: As precise as possible (listen: Thomson 253H has not requested anything, so TWR is offering the prime information which the pilot needs now): 

TWR: Thomson 253H, roger, all runways are available for landing, surface wind 070° at 5 knots.

3. ATC warns all other stations ans waits. the pilot is "in command", he will call.

Acft: Manchester, Mayday 253H

4. Pilot has called, now TWR offers solutions.

TWR: Mayday 253H, all runways are available. If you wish, vectors, or continue visually for 06 left or right.

5. Now the pilot says his intentions:

Acft: Manchester, we are heading in a northwesterly direction and trying to establish on the 85° inbound radial towards Wallacy, climb to 3500 ft and then advise you about our further intentions.

6. Manchester Tower reads back and done. If the pilot does not want anything else, then TWR keeps quiet. He knows that the pilot will fly a controlled approach via a VOR (Afterwards we will see that his right engine is dead while the left one runs normally - far enough for a modern aircraft to take off and climb). Shortly later he makes the offer to land in Liverpool - which is turned down - the pilot is happy with returning to Manchester. Then TWR reports runway 06R closed because of debris - the aircraft has lost parts of its engine. This is an order to the pilot - he should not land on a closed runway. A short time afterwards, the aircraft lands orderly on 06L.

Engine Fire

... has the same consequences as engine failures - pilots cut off fuel immediately and won't re-ignite it. Only, that life in the air might be much shorter, if the fire does not stop.

Engine Failure at approach - Triebwerksversagen beim Landeanflug

Partly engine failure

Modern aircraft can take off with half of their engines dead - let alone landing. With one engine dead (or thrust reduced), aircraft will usually receive priority landing. If the engine failure happens with ILS established, the aircraft will likely loose it (AP disconnect, plane drifts or drops).

For ATC this means: Listen what the pilot wants to do: go-around? continue visually? ... and clear the way.

The only thing you want to avoid is go-around: Full thrust on only one engine makes nasty jaws which could cause an aircraft to drift and drop. Turns are difficult or impossible. For ATC this means: clear the airspace around and in front, as an aircraft under this condition can drift into any direction.

Total engine failure

For ATC, this means: This aircraft will come down very quickly. In this situation, workload in the cockpit is enormous, as they try to re-light the engines. ATC's job is to analyse every option to reach ground safely and offer them in sequence of relevance. A good reference is the US Airways Flight 1549 - the famous Hudson River landing: how short and efficient communication can be under these severe circumstances.

(Very) low fuel

If fuel is gone, then this is the same situation as a complete engine failure. If such a situation is possible or even likely (=if it could happen within the distance between aircraft and airport), then the aircraft has to be routed to the nearest runway threshold. Again - the pilot decides, bearing in mind that steep turns cost altitude. If no runway can be reached, then anything like it is better than nothing - a taxiway maybe. As ATC, you could indicate emergency landing spots in reach. See this video on the Air Transat incident as a good and this video on the Avianca 52 crash as a deadly example.

Instrument Failure

The question is: Which instrument fails? It could be any. Again: Pilot will tell what he means. Vital instruments and best reaction:

• Pitot: This measures speed, which is vital to stay in the air. The pilot will most likely ask you to report his speed in regular intervals or to monitor a speed band he has to stay ind. Bear in mind that you can only tell ground speed and that in high altitude, this considerably differs from IAS (which the pilot needs). Report what you know - ground speed. The pilot knows altitude and temperature and can do a rough guess of his IAS. If speed monitoring fails and no ATC around can report it (over the ocean), then [this] happens.
• Directional gyro: This is the "compass" of the aircraft. Most likely, you as ATC will notice first that the aircraft goes into strange circles. If the pilot has no backup compass to look at, then the ground is the only possible visual reference. ATC can approve descent to minimum altitude. Then, incremental turns can be issued: "turn 20° left from present heading" - until runway is in sight.
• FMC: Easy. Vector him.
• VOR receiver: This means that the aircraft cannot fly an ILS - clear him/her for visual approach. If there are clouds: divert, or if there is no choice, vector him and report altitude at regular intervals.
• Altimeter failure is likely to become a problem when it goes along unnoticed, as autopilot heavily relies on this. Most likely you as VATSIM controller are to know it first. In real life, this is not the case, as plane altitude is received via Transponder, and the transponder gets his information - from the altimeter. This video shows what could happen. As controller, you have to rely on what the pilot requests.
• All Avionics: Likely to happen if all power fails on the aircraft. Count on the pilot to have the ground as the only visual reference. He is likely to ask you for relative directions (o'clock positions to turn to). In real life, altimeter is likely to fail too, and the squawk does not transmit correct altitude signals to ATC - don't report altitude unless you can see the aircraft or have other means to tell it (like precision radar). At VATSIM, you won't have that problem (but you can simulate it).

Landing gear failure

• If landing gear fails to retract after takeoff, then this is a pan-pan situation. Crew can continue normally, but with reduced speed and more fuel consumption - crew will recalculate if they need a diversion. For ATC this means to accomodate crew's wishes.
• A broken tyre could prevent the aircraft from retracting the gear. Landing with a flat tyre is already an emergency, as the aircraft could become unstable after touchdown. For ATC this means: Clear the runway, chase away aircraft on ground near the runway, increase spacing behind the landing aircraft and alert fire services.
• If the aircraft fails to expand gear, then the situation is more critical. The only thing pilot see is their warning lights (they can't get out and see). This will occur in the last 10miles final. First reaction is a go-around to gain time to analyse the problem. In real life, the pilot performs a low approach and technicians look at the aircraft from below. He/she will also perform a series of steep manoevers high over ground to make the gear fall into place and needs space for it. If gear fails to expand properly, then an attempted landing is imminent (at some point, the aircraft must land). This is an emergency. For ATC: Clear the runway and adjacent taxiways, increace spacing behind and alert fire services. The pilot will decide, how to land: with partly expanded gear or on the stomach (most handbooks have a procedure for landing with partly expanded gear. Stomach landing is not an option usually unless you land on water).

Multiple emergencies

There is nothing more challenging than multiple emergencies, as you as controller have to rank its severity and propose measures that all of them can accomodated. If emergencies collide, you have to set priorities:

• Aircraft which need to come down soon have priority to those which can still fly for a few minutes (heart attack is first compared to hydraulics failure).
• More people saved is better than less (full engine failure has priority compared with a passenger with heart attack).

Nothing is heavier than taking these decisions. Let's be happy to fly online.

Further reading

A [brochure by the British Air Traffic Authorities] is an excellent reference for controllers.