No TCAS, no flight

[Timmy Tomkins]

Horizon in the US unable to use exemption common to other airlines.

https://www.seattletimes.com/business/boeing-aerospace/unlike-other-airlines-faa-wont-let-horizon-fly-without-anti-collision-system/

[BFSGrad]

"FAA policy has long allowed airlines up to three days’ leeway during which they can continue to fly a plane with an inoperative system.

But Clint Laurie, the FAA’s principal operations inspector at Horizon, supported by his team that oversees the airline, is enforcing a strict interpretation of the TCAS regulation."

Is Laurie an FAA employee or is he a Horizon employee designated by the FAA as an inspector? If the former, that would seem to be a problem; i.e., he’s implementing an operational restriction that’s inconsistent with the policy of his employer. If the latter (more likely), he’s simply implementing an operational constraint that’s more restrictive than existing FAA regs/policy, but is doing so under the authority of Horizon.

[John Marsh]

This also from the Seattle Times article:
In early February, according to a document viewed by The Seattle Times, Horizon sent a memo to flight maintenance crews informing them of this. The jets can fly to another base for repair, but not with passengers aboard. This notice was “effective immediately.”
SLF here. I'd appreciate that as a prospective Horizon customer. It begs the question: what about the safety of other operators' passengers? Or flight crews, including those at Horizon?

[mnttech]

Is Laurie an FAA employee or is he a Horizon employee designated by the FAA as an inspector?
By title, I'm going to assume FAA employee.
The FAA Master MEL, depending on installed avionics equipment, states 2 flight days (Cat A) or 3 days (Cat B), Note, the Horizon MEL may be more restrictive.
14 CFR 121.356 says you have to have one. I wonder if the "exemption common to other airlines" are to the Rule, and Horizon just does not have that, hence the issue.

[Capn Bloggs]

That's ridiculous. How can a lone FAA inspector unilaterally apply, to only one airline, a rule that overrides the legal document that we all operate to, the MEL?

By all means change it but any change must apply to all operators.

SLF here. I'd appreciate that as a prospective Horizon customer.
Perhaps you shouldn't go flying, John. And remember that the other aircraft will have a serviceable TCAS which will issue a avoidance manoeuvre to the other crew if needed (unless of course it too is operating under the MEL with a U/S TCAS; highly unlikely). TCAS is not required in both aircraft.

[Zar_1]

Considering how crowded our skies are today, I've always felt strongly that systems like TCAS (and preferably also GPWS/EGPWS) must always be operative.

[Abbey Road]

Is there any chance Horizon have, perhaps, been loosely interpreting some rules, and are now being hit with a crackdown to make them toe the line? Just wondering.

[albatross]

Considering how crowded our skies are today, I've always felt strongly that systems like TCAS (and preferably also GPWS/EGPWS) must always be operative.

Really? So if your magic computer breaks at some remote location miles from nowhere what do you do to get someplace to get it fixed?

[Zar_1]

Really? So if your magic computer breaks at some remote location miles from nowhere what do you do to get someplace to get it fixed?


What I implied was, that TCAS INOP should be a non-dispatchable condition... of course, a ferry flight would be permitted... but really though, considering how 'see and avoid' is clearly deficient at modern airliner speeds, and under high workloads, is it really safe to dispatch a flight without a working EGPWS/TCAS ?

Aviation has reached such heightened levels of safety, owing a lot to the automation and safety systems, which serve to significantly enhance Sit. Awareness... and permitting revenue flights without them, IMO, is a mistake.

[FlightDetent]

Considering how crowded our skies are today, I've always felt strongly that systems like TCAS (and preferably also GPWS/EGPWS) must always be operative. Moreover, the Earth is littered with ground surfaces these days.

[tdracer]

That's ridiculous. How can a lone FAA inspector unilaterally apply, to only one airline, a rule that overrides the legal document that we all operate to, the MEL?

By all means change it but any change must apply to all operators.
I wholeheartedly agree - it is blatantly unfair to hold one operator to a higher standard than everyone else.
What I implied was, that TCAS INOP should be a non-dispatchable condition... of course, a ferry flight would be permitted... but really though, considering how 'see and avoid' is clearly deficient at modern airliner speeds, and under high workloads, is it really safe to dispatch a flight without a working EGPWS/TCAS ?

Aviation has reached such heightened levels of safety, owing a lot to the automation and safety systems, which serve to significantly enhance Sit. Awareness... and permitting revenue flights without them, IMO, is a mistake.
So, if you're going to allow a ferry flight - if that results in a mid-air due to the inop TCAS, the other aircraft - presumably a revenue flight with a full load of passengers - is also going to crash. How are you going to prevent that if it's so unsafe to have a flight without a working TCAS?

[fdr]

I wholeheartedly agree - it is blatantly unfair to hold one operator to a higher standard than everyone else.

So, if you're going to allow a ferry flight - if that results in a mid-air due to the inop TCAS, the other aircraft - presumably a revenue flight with a full load of passengers - is also going to crash. How are you going to prevent that if it's so unsafe to have a flight without a working TCAS?

It is not unsafe to fly without TCAS. For there to be an issue, the other aircraft would have to not have TCAS as well, like the piper cub on floats crossing the LLZ outside of the veil in PANC... who doesn't need an electrical system. I own a jet that was used for TCAS testing, and the failure rate of TCAS is enough to strand pax in various parts of the world, by the number of times the system was swapped out for repair.

[fdr]

What I implied was, that TCAS INOP should be a non-dispatchable condition... of course, a ferry flight would be permitted... but really though, considering how 'see and avoid' is clearly deficient at modern airliner speeds, and under high workloads, is it really safe to dispatch a flight without a working EGPWS/TCAS ?

Aviation has reached such heightened levels of safety, owing a lot to the automation and safety systems, which serve to significantly enhance Sit. Awareness... and permitting revenue flights without them, IMO, is a mistake.

Is there a particular accident you are referring to, where a TCAS or TAWS system was MEL'd and the crew had a bad day? The primary means of traffic and terrain separation (well, departure can be slightly different) is ATC, then alerting, then a pilot that is competent and awake.

[AerocatS2A]

Considering how crowded our skies are today, I've always felt strongly that systems like TCAS (and preferably also GPWS/EGPWS) must always be operative.
If I was getting TCAS RAs everyday I might agree with you, but the only RAs I’ve ever had were “nuisance” alerts where one aircraft was levelling off 1000’ above or below another. There’s also the point that, unlike doing the Tango, it only takes one to TCAS RA, so provided the MEL alleviation is short enough that the chances of coming across another aircraft operating under the same MEL are low enough, I’m comfortable with it as it is.

[safetypee]

Which equipment is used in this aircraft ?

A stand alone ACAS / TCAS; or a combined system, T2CAS which has both ACAS and TAWS functions in one 'box'.
Which ever, how does the MEL provide alleviation for the different systems ?
Does the MEL differentiate between hardware (box) and the function; e.g. power supply or sensor input.

Anyone recall the saying "Two TCAS North of Darwin".
Reflecting a safety culture (and perceived hazard) as much as the system availability.

[Zar_1]

Is there a particular accident you are referring to, where a TCAS or TAWS system was MEL'd and the crew had a bad day?

The tragic mid air between that Embraer and the 738 is an example, not to mention the dozen or so near misses that TCAS has prevented.

I do agree, yes, Aviation wouldn't be unmeasurably unsafe if it weren't for TCAS. I'm just saying... aviation is all about reducing risk, and allowing a flight sans TCAS/GPWS IMO isn't a great idea.

PS. I totally forgot to mention, sorry, that IIRC, the Transponder went INOP mid flight

[tdracer]

The tragic mid air between that Embraer and the 738 is an example, not to mention the dozen or so near misses that TCAS has prevented.

I do agree, yes, Aviation wouldn't be unmeasurably unsafe if it weren't for TCAS. I'm just saying... aviation is all about reducing risk, and allowing a flight sans TCAS/GPWS IMO isn't a great idea.

PS. I totally forgot to mention, sorry, that IIRC, the Transponder went INOP mid flight

Zar, I don't think you really understand the purpose of the MEL. In a perfect world, every aircraft would takeoff with every system working perfectly, pilots would always be well rested and alert, and ATC would never make a mistake.
The world's not perfect. Things break, pilots fly fatigued, human pilots and ATC make mistakes. S*** Happens. IF we required every commercial aircraft to have every system fully functional in order to carry passengers, the whole aviation industry would grind to a halt - Delays and Cancellations would skyrocket, people would be completely unable to count on it. Commerical aircraft - heck the entire aviation industry - is designed with high levels of redundancy. That's the major reason why aviation is so safe. Sometimes some piece of that redundancy fails - a component fails, a human makes a mistake - but the redundancy takes over and keeps things safe.
Considerable effort goes into development of the Master Minimum Equipment List - MMEL. Almost without exception, items on the MMEL allow for a failure that results in a loss of redundancy. That loss of redundancy is evaluated - how serious is that redundancy loss, what's the consequence of the next failure, what's the likelihood of the next failure. Then the airframer and the regulators get together and determine what is acceptable without a measurable impact on safety - and for how long. It's accepted that there will be a tiny reduction in the margin of safety, but the effect on the overall safety is negligible. It allows aircraft to keep flying, to get back to a maintenance base where it can be fixed properly. And remember, the pilots get the final say - if they decide it's not safe to dispatch with a particular system (or systems) deactivated via the MEL, they can say NO (not saying there won't be pressure to go, but remember the pilots are usually the first ones at the scene of a crash so they have a vested interest to refuse something unsafe).
Dispatching an aircraft with an INOP TCAS reduces the redundancy of the system, but you still have two more levels - ATC and TCAS on the other aircraft - before it becomes unsafe. It's been judged acceptable to do that for three days.

Oh, BTW, Überlingen was one of the worst midairs in history. Both aircraft had fully functional TCAS systems at the time of the crash. As I said, somethings :mad: happens.

[ATC Watcher]

Well said tdracer.
To add a bit of first hand ATC knowledge, actually when we study Mode S downlinks , we find that over half of the RAs generated by TCAS are not followed correctly, some of them not at all, and even sometimes flown in the opposite direction ( which was the case in Ueberlingen mentioned by tdracer. )
And then there are the TA-only pilots , for some obscure reasons there are still pilots that fly in TA mode only , which means they are not protected . In Europe ( and suspect similar figures in the US) we find aircraft involved in incidents ( airproxes) with one aircraft being on TA mode
And I am not talking about close parallel approaches where you may switch to TA only.
From a reecent Eurocontrol safety bulletin :
The examination of radar data collected in core European airspace shows that on average 50 flights a day were conducted in TA-only mode (that excludes aircraft performing parallel approaches).
Then ,taking all this into account, adding your 2 or 3 aircraft flying with an INOP TCAS for max 3 days is not treally that releveant.
Relying solely on TCAS to keep you safe is not a guarantee you will never meet someone one day .

[Zar_1]

Dispatching an aircraft with an INOP TCAS reduces the redundancy of the system, but you still have two more levels - ATC and TCAS on the other aircraft - before it becomes unsafe. It's been judged acceptable to do that for three days.


Yes, sorry, I totally forgot that an INOP TCAS wouldn't lead to a total loss of traffic awareness (we do have the ATC and TCAS on the other ac).

[mnttech]

All,
The FAA Master MEL (MMEL), depending on installed avionics equipment, states 2 flight days (Cat A) or 3 days (Cat B), Note, the Horizon MEL may be more restrictive.
MMEL's are written as far as I know against the aircraft certification rules, in this case 14 CFR 25, or the EASA equal. The FAA team that writes the MMEL does not know if you are going to operate the aircraft Part 91, Part 121, Part 135 etc. Each operator has to then take the MMEL, and update it for their operation, and submit it to the FAA for approval.

14 CFR 121.356 says you have to have one. I wonder if the "exemption common to other airlines" are to the Rule, and Horizon just does not have that, hence the issue.
That is an operational rule and without the exemption (if that is the case) Horizon cannot fly and the inspector is correct. Each airline has to apply for the exemption as far as I know.

[Capn Bloggs]

The FAA does not know if you are going to operate the aircraft Part 91, Part 121, Part 135 etc.
Of course they do (or at least, here they do). We have an approved MEL for our operation. Either operation for three days with the TCAS inop is in the MEL and therefore approved, or it's not. You can't have some wally in an out-office saying "the MEL doesn't apply, you have to have the TCAS working for every flight" because the base rule says you need it.

[AerocatS2A]

Of course they do (or at least, here they do). We have an approved MEL for our operation. Either operation for three days with the TCAS inop is in the MEL and therefore approved, or it's not. You can't have some wally in an out-office saying "the MEL doesn't apply, you have to have the TCAS working for every flight" because the base rule says you need it.
Not only that, but the base rule will say the equipment must be fitted to the aircraft, but there will be another rule allowing the development of an MEL that permits the fitted equipment to be unserviceable for a period of time. I haven’t checked FAR 121 but 135 is done that way and I’m sure 121 will have a similar clause.

[Uplinker]

Just for the nervous flyers; over the last 20 years; flying regularly in VERY busy airspaces, including the London FIR, Paris and New York, I have only ever had a handful of TCAS TAs, (Traffic Advisories, i.e. warnings of potentially conflicting traffic, but no recommendation to change flight path), but NEVER any RAs, (Resolution Advisories, i.e. recommending action to be taken to avoid actual conflicting traffic).

We do get them in every 6 month Simulator check, to practice the avoidance manoeuvre(s), but I have never had one for real.

Air traffic control in 'civilised' places is very good and safe :ok:

PS, Most TAs are caused by your, or other, aircraft climbing or descending more rapidly than 'normal' but nevertheless to a safe flight level above or below the conflicting traffic; so there is no collision risk. Such TAs are easily and quickly resolved by simply reducing their (or your) vertical speed.

TCAS is an extra safety layer on top of the many other tried and tested ATC procedures and systems.

[mnttech]

I have updated my post
The FAA team that writes the MMEL does not know if you are going to operate the aircraft Part 91, Part 121, Part 135 etc. Each operator has to then take the MMEL, and update it for their operation, and submit it to the FAA for approval.

[Capn Bloggs]

Each operator has to then take the MMEL, and update it for their operation, and submit it to the FAA for approval.
If a company even hinted it was going to run a high-capacity pax operation ie Part 121 op the FAA would be all over it like a rash well before even the MEL-approval stage. And as you say, the MEL has to be approved for that company aircraft.

Unless Horizon is/was operating without an approved MEL, or their MEL did not permit the normal 3 day grace period for TCAS rectification, that FAA inspector is out of line by unilaterally stopping them from operating with a U/S TCAS. I suppose we'll never know unless we get a copy of their approved MEL for the 175.

[Capn Bloggs]

Some interesting scenarios here:

https://www.airliners.net/forum/viewtopic.php?f=3&t=1483455

[mnttech]

I suppose we'll never know unless we get a copy of their approved MEL for the 175.
Something I was about to post, thank you

Along with what equipment is on the aircraft, as you noted the in the airliner.net link the background between the two groups, and what the writer of the report meant.
A quick check here does not show any of the majors with an posted FAA exemption to 121.356.
https://aes.faa.gov (https://aes.faa.gov/)

[Capn Bloggs]

I doubt whether an exemption is granted to anybody to allow use of an MEL. I can't find any reference to US operators, but have a look at the extensive information on MELs for foreign operators of US registered aircraft:

https://www.ecfr.gov/current/title-14/chapter-I/subchapter-G/part-129/subpart-A/section-129.14

Clearly, the approval for and use of an MEL is embedded in the regs and presumably not an "exemption".

I also severely doubt that there are any 121 operators in the US that don't have an MEL that allows 3 days grace for TCAS.

Something fishy going on.

[albatross]

Just an aside:
in “Olden Daze” it was quite normal when using Maps, ADF, VOR ect to operate betwixt Greater Moosepoop and Little Moosepoop and see no other traffic.
Then along came GPS and we would see a lot more traffic because we were all exactly on track

There was an unwritten rule that you always flew on the right side of any river, lake, road, dogsled track, valley, railway or power-line for avoidance of opposite direction traffic. (“Humm 200 overcast 1 mile in rain and fog! Weather’s too bad for IFR! Guess we’ll have to go VFR”)
.

[AerocatS2A]

Something I was about to post, thank you

Along with what equipment is on the aircraft, as you noted the in the airliner.net link the background between the two groups, and what the writer of the report meant.
A quick check here does not show any of the majors with an posted FAA exemption to 121.356.
https://aes.faa.gov (https://aes.faa.gov/)You don’t need an exemption, you just need an MEL in accordance with 121.628. That is the method by which the FAA approve an aircraft to be flown with inoperative equipment. An exemption would be required for something that was not covered by the MEL.

https://www.ecfr.gov/current/title-14/chapter-I/subchapter-G/part-121/subpart-U/section-121.628

[mnttech]

And around and around and around and around we go...
You don’t need an exemption, you just need an MEL in accordance with 121.628.
https://www.ecfr.gov/current/title-14/chapter-I/subchapter-G/part-121/subpart-U/section-121.628§ 121.628 Inoperable instruments and equipment

(b) The following instruments and equipment may not be included in the Minimum Equipment List:

(3) Instruments and equipment required for specific operations by this part.
Does the requirement for TCAS under 121.356 fall under 121.628(b)(3)?
We don't know....

[AerocatS2A]

And around and around and around and around we go...

Does the requirement for TCAS under 121.356 fall under 121.628(b)(3)?
We don't know....
That would cover specific operations, like ETOPS. The TCAS is a general requirement of 121, it’s not related to a specific operation.

[AerocatS2A]

And around and around and around and around we go...

Does the requirement for TCAS under 121.356 fall under 121.628(b)(3)?
We don't know....
How do you read 121.303? https://www.ecfr.gov/current/title-14/chapter-I/subchapter-G/part-121/subpart-K/section-121.303

The TCAS isn’t listed.

[mnttech]

That would cover specific operations, like ETOPS. The TCAS is a general requirement of 121, it’s not related to a specific operation.OK, where is “specific operations” defined? Not disagreeing, but in a quick search I don’t see the term defined in 121 or 14 CFR 1. Would Night IFR over water be a specific operation?
How do you read 121.303? https://www.ecfr.gov/current/title-14/chapter-I/subchapter-G/part-121/subpart-K/section-121.303
The TCAS isn’t listed.
Not sure, the only thing I noted is the TCAS rule is a weight and class controlled while 121.303 is for all operations. There appear to be a couple other rules that drop out too.
We can play rule poker all day long, but those that work for the FAA have a little more pull. I have not dug into any of the policies, orders or Memo's (which may set policy) that the FAA has that defines the rules (law). Nor have I looked at any of the AC's that provide guidance to reaching the rule.

[AerocatS2A]

OK, where is “specific operations” defined? Not disagreeing, but in a quick search I don’t see the term defined in 121 or 14 CFR 1. Would Night IFR over water be a specific operation?

Not sure, the only thing I noted is the TCAS rule is a weight and class controlled while 121.303 is for all operations. There appear to be a couple other rules that drop out too.
We can play rule poker all day long, but those that work for the FAA have a little more pull. I have not dug into any of the policies, orders or Memo's (which may set policy) that the FAA has that defines the rules (law). Nor have I looked at any of the AC's that provide guidance to reaching the rule.
Do you fly? What does your MEL say? We only need any US 121 operator to be shown to have TCAS on their MEL to demonstrate that it is allowed by the rules. It’s on my MEL for the A320 but it’s not a US company, so a bit irrelevant. Ours is even more lenient, running up to 10 days except for specific operations including uncontrolled airspace and NATS airspace with half degree track spacing.

[mnttech]

Do you fly?
Not any more, the B707 I crewed was retired long before TCAS was approved, and the 14 CFR 135 operator I work for does not need TCAS.
I would like to see a USA 121 MEL too.
But that is kind of the drift of the whole thread, do we have a rouge inspector, or has everyone not been reading the rules?

[tdracer]

.
But that is kind of the drift of the whole thread, do we have a rouge inspector, or has everyone not been reading the rules?

The airframer publishes the Master MEL (BTW, the MMEL has different provisions for ETOPS and the like). Each operator then takes that MMEL and 'customizes' as necessary for their operation and gets that approve by the local regulator(s) - usually their MEL is identical (or nearly identical) to what the airframer put out. An operator can put in more restrictive provisions into their customized MEL, but it is very, very rare that an operators MEL is less restrictive than the MMEL (and it is very messy to get that approved by the regulators).

[BFSGrad]

The Seattle Times editorial board has produced this hot mess of an opinion piece, concluding that the 3-day TCAS grace period is a “huge risk.”

FAA’s priority should be passenger safety, not airlines’ profits (https://www.seattletimes.com/opinion/editorials/faas-priority-should-be-passenger-safety-not-airlines-profits/)

[tdracer]

The Seattle Times editorial board has produced this hot mess of an opinion piece, concluding that the 3-day TCAS grace period is a “huge risk.”

FAA’s priority should be passenger safety, not airlines’ profits (https://www.seattletimes.com/opinion/editorials/faas-priority-should-be-passenger-safety-not-airlines-profits/)

And they are being royally roasted in the comments for talking about something they know nothing about.

[BFSGrad]

And they are being royally roasted in the comments for talking about something they know nothing about.Hah! Hadn’t read the comments. A well-deserved weenie roast. These days it’s not unusual for the comments associated with a news article to be more informative than the article itself. Both the original Gates’ article and the editorial do a poor job of quantifying the risk of an inoperative TCAS.

[ATC Watcher]

Some great comments indeed :ok:. Except when they mention Uberlingen , but one cannot expect everyone to have read the report ..

[212man]

The Seattle Times editorial board has produced this hot mess of an opinion piece, concluding that the 3-day TCAS grace period is a “huge risk.”

FAA’s priority should be passenger safety, not airlines’ profits (https://www.seattletimes.com/opinion/editorials/faas-priority-should-be-passenger-safety-not-airlines-profits/)
“Aircrafts”! That’s just basic English - no aviation knowledge required.

[AerocatS2A]

“Aircrafts”! That’s just basic English - no aviation knowledge required.
It’s becoming more common. I notice it most with non native English speakers and more and more with native speakers. It was even in Microsoft flight sim FFS!

[rigpiggy]

Considering how crowded our skies are today, I've always felt strongly that systems like TCAS (and preferably also GPWS/EGPWS) must always be operative.

In northern Alberta seat recently a B412 cut off the bambi bucket on another helo during fire suppression. Why TF ADS-B isn't required by government for these contracts? The Kmax has it, one other he'll has tcas, all the other ones are running around on reporting positions. Wtactualf. Stay safe everyone

[Capn Bloggs]

In northern Alberta seat recently a B412 cut off the bambi bucket on another helo during fire suppression. Why TF ADS-B isn't required by government for these contracts?
​​​​​​​You've obviously never used TCAS. Never designed for close-in collision avoidance. In that scenario, I suspect it would be going off continuously and drive the crews mad.

[Big Pistons Forever]

This makes me wonder about regulatory capture. There is huge pressures on the regulators to approve MEL’s because of the potential cost savings of not having an airplane grounded when something breaks. Personally I think allowing a 3 day window on an inoperable TCAS should never have been approved for any operator.

[Capn Bloggs]

Hang on. These are highly sophisticated bits of gear. Of course they are going to go on the blink occasionally. And as already stated, TCAS will work even if the other aircraft only has a transponder ie it's TCAS system is U/S. And aircraft are not permitted into most controlled airspace unless their transponder is working. The 3 day MEL has been in place for decades, on all sorts of aircraft systems. If the FAA want to change the rules, fair enough, but it must apply to all operators.

[Ollie Onion]

Just finished a full day flying in an aircraft with u/s TCAS and the MEL clearly states you have 72 hours grace. How can you justify applying MEL documents differently depending on the operator? TCAS is a great safety feature but shouldn’t be a no go.

[tdracer]

This makes me wonder about regulatory capture. There is huge pressures on the regulators to approve MEL’s because of the potential cost savings of not having an airplane grounded when something breaks.
That's why the manufacturers create the "Master" MEL (MMEL). Operators can go more conservative than the MMEL with their own MEL, but it's nearly impossible for them have a more permissive MEL than the Master.

[Big Pistons Forever]

That's why the manufacturers create the "Master" MEL (MMEL). Operators can go more conservative than the MMEL with their own MEL, but it's nearly impossible for them have a more permissive MEL than the Master.

Yes but the MMEL is approved by the regulator. A TCAS RA only works properly if both aircraft have an operational TCAS. I don’t think it is reasonable to have a device that has demonstrably prevented many midair’s inoperable in an airplane.

This is putting money ahead of safety

[tdracer]

So you're going to ground an aircraft at a remote location if TCAS has a glitch? Because if it's unsafe, it's unsafe for a ferry flight as well (after all, the aircraft it collides with doesn't care if it's a ferry flight).
Go back and read the discussion on the first page of this thread.

[Capn Bloggs]

A TCAS RA only works properly if both aircraft have an operational TCAS.
What do you mean, "properly"? The only advantage of having TCAS in each is that they talk to each other eg I'll go up, you go down". The system still works fine with only one TCAS.
​​​​​​​
What about all those lighter types that have a transponder but no TCAS, mixing it with airliners in the terminal areas?

[ATC Watcher]

Yes but tha device that has demonstrably prevented many midair’s inoperable in an airplane.

You have any Stats / data to demonstate that statement ? Because we don't , We have data that demonstrate that the drastic redictions in mid-airs is due to Short term conflic alerts being introduced widely in ATC . To us TCAS caused f more loss of separation ( I said separation, not collision risks) than saving the day. .
To close the discussion on the ACAS u/s 72 hours grace or MEL conditions , remember TCAS is ony mandatory for large aircraft , and even then excluding State aircraft and military, There are far , far more flying "things" flying around which are not equipped , not even with transponders ( e.g gliders) which are plenty in class E airpace.on a good day. As I said before , TCAS is a nice piece of equipment adding one safety layer when working, , but even when working is no garantee at all you will not hit anybody else. So no problem for us if it is not working for 3 days on a single aircraft.

[Capt Pit Bull]

People should be more worried about tokenistic TCAS training for flight crew rather than the slim possibility TCAS might be inop.

[Nugget90]

MEL Provisions - Historical Background to TCAS/ACAS II Development

As there seems to be quite a lot of interest regarding MEL provisions whereby, as it is mentioned, flights might depart without the TCAS (essentially ACAS II) being fully serviceable, I would just like to provide some background information as to why this can be permitted. In the late 1980s when it was starting to become apparent that TCAS would be adopted as a potential aid to alert pilots of impending undue proximity to another aircraft I, as a Flight Operations Inspector in the UK Civil Aviation Authority, was tasked with representing the Authority's interests from a transport aircraft pilot's perspective. Consequently I established contact with the FAA, 'flew' an early version in a flight simulator, and observed how the equipment worked from within the flight deck of a Piedmont Airline aircraft transiting between two aerodromes. (The FAA also very kindly let me fly a breadboard version of what was being developed as equipment to satisfy an ACAS III standard, that is to say equipment where both vertical and horizontal advisories might be posted: it worked very convincingly, but as there seemed to be no identifiable need for this level of alerts it didn't go much further at that stage.)

In the UK, an ACAS (for those who don't know this acronym, this stands for Aircraft Collision Avoidance System) Operational Safety Working Group was set up, involving air traffic controllers, scientists, military interest and of course UK airline and CAA pilots, to understand what TCAS II - as a solution for ACAS II (ie the vertical-only advisory generating equipment) might do, what operational guidance might be published, how aircrews might be trained when their aircraft had this equipment installed and certificated, and what air traffic controllers might expect to observe.

Amongst many topics that were discussed was how, if there should ever be a requirement that a certain class of aircraft, such as aeroplanes over a certain mass, helicopters, etc. must have TCAS II installed, flights might or might not be permitted to take place if the system became inoperable. Whilst one point of view was that the aircraft should not be allowed to get airborne, or if it were to fly should remain clear of controlled airspace and remain in VMC (Visual Meteorological Conditions), others pointed out that this could be quite impractical in airline operations and that concessions should be made via the MEL (Minimum Equipment List). After discussion it was agreed that if, for example, an aeroplane normally based in the UK were to find its TCAS equipment unserviceable when away from base, somewhere in Australia, possibly, it would be sensible, taking into account all the other means by which separation between aircraft was normally applied, to permit this aeroplane to fly for a limited period sufficient for it to land at a place where a repair could be made. But this period of grace would be subject to certain conditions such as a maximum number of days, not to proceed beyond an airfield at which a repair could be made, and (later on when separation standards were reduced) not to fly within certain airspace in which serviceable TCAS was prescribed. It is worth remembering that 30+ years ago aeroplanes transiting across the world would make more stops en route that they would today, and that reserves of spares that might be needed to rectify an inoperable TCAS - with the emphasis on the 'S' for System - might not be readily available.

This decision was agreed within the Safety Working Group, was later taken up by the Joint Aviation Authorities (when I was the UK representative) and by ICAO Flight Operations Panel (ditto). Safety is the product of managing risks, and I'm not aware that flight operations have suffered unduly where flights have taken place with inoperable TCAS. It is worth remembering that if two aircraft normally required to have TCAS are converging and one has inoperable TCAS, it is highly unlikely that the other will also have its TCAS equipment inoperable.

I hope that this little essay helps clarify how it came about at the outset that flights by aircraft required to have TCAS installed might be permitted to get airborne with the equipment unserviceable, and that this concession has been agreed on a universal basis subject, as always, to conditions.

[Miles Magister]

Guys,
There is a lot of mention about the FAA MMEL above but this is not a legal operational authority. You must check the AIP for the airspace you are going to operate in and comply with national legislation. Some countries do indeed allow a 10 day alleviation, some a three day and some do not allow any alleviation at all where ACAS must be serviceable.

Check the national AIPs .

MM

[HOVIS]

Nugget90.
Very informative, thankyou.

For everyone else worried about TCAS MEL allowances, best you don't read the rest of the MEL! 😁

[AerocatS2A]

Guys,
There is a lot of mention about the FAA MMEL above but this is not a legal operational authority. You must check the AIP for the airspace you are going to operate in and comply with national legislation. Some countries do indeed allow a 10 day alleviation, some a three day and some do not allow any alleviation at all where ACAS must be serviceable.

Check the national AIPs .

MM
Yes of course. The context of this discussion is operating in FAA airspace so that's why it's all about the FAA MEL.