View Full Version : 3 AUTOPILOTS?????

J32 8069
12th Jan 2002, 00:40

Max Flyup
12th Jan 2002, 01:21
The F100 is a Cat3c aircraft. To be so it must have 3 autopilots - if it had 2 and 1 was wrong there would be no way of knowing which was correct. By having 3 it can always compare 2 out of the 3 and 'out vote' the faulty channel.

At least that is my understanding....

12th Jan 2002, 01:35
I believe that to be true.

Also, just for fun, AP1 is usually used on the 1st leg, say, of a 2 leg trip and AP2 on the way back.

12th Jan 2002, 05:02
True enough.

Except in my mob, we only use the centre autopilot for flight.

All THREE A/P's lock in for an Automated landing...assuming they all work. If all 3 do work, then you can operate to CAT3B minima. If only 2 work, you can fly to CAT3A or CAT2 minima.

Or something like that. In essence, the more functioning autopilots you have, then the lower the minima you can fly to.

12th Jan 2002, 10:16

13th Jan 2002, 08:16
Three autopilots or AFCS give "Fail Operational" conditions - a faulty channel is automatically detected by comparison to the two serviceable channels. The two channels then become "Fail Passive", where a disagreement between the two indicates a fault but cannot discriminate which channel is faulty. In general, you can perform Cat III autolands with a Fail Operational system but are restricted to Cat I or II with Fail Passive.

Most of these current autopilot models are ground tested using BITE and the old practices of full autopilot checks lasting a couple of hours are gone. It is therefore necessary to check each channel by regularly using it in flight, since much of the automatic internal monitoring is only performed in flight conditions. Boeings have a test called 'MCDP Test 40' that appears to be a full check out, but this is a fallacy. That is the reason why operational policy is set to use different autopilots on different sectors - to ensure that they will be fully functional when you really need them. You should ideally be using all three channels regularly. Someone mentioned using only the centre channel: if you are on a Boeing (others too, but I'm mainly familiar with Boeings) and each pilot uses the on-side Flight Director then all three channels are being monitored, but you should still check the servo feedback loops regularly by using channels one and three.

Through difficulties to the cinema

14th Jan 2002, 20:03
So 2 out of 3 autopilots are redundant ???

Don't they have a union ... <img src="mad.gif" border="0">

14th Jan 2002, 21:00
Redsnail - I fly a 3 autopilot aeroplane but the company are still trying to make me redundant <img src="confused.gif" border="0">

14th Jan 2002, 22:17
In "Fail Operational" each autopilot uses a different electrical source so that guidance isn't lost at a critical time. All 3 autopilots are engaged in CAT3B operations, so if you have a single electrical failure close-in, you can still complete the approach, ie the remaining autopilots will flare, touchdown and track centerline.

Localiser Green
14th Jan 2002, 23:07
But the Airbus family (A320/330/340) have 2 Autopilots (1 and 2) and are still CATIIIB/C certified.

I belive also the 737 only has 2 - A and B?

So are their autopilots just 'better' than the likes of the 757/767?

15th Jan 2002, 00:39
Because they, like the old Tristar (guvnor!!!) have a dual dual system....2 A/Ps but each has a command and a monitor channel...so in effect 4 A/Ps!!! Well almost..

15th Jan 2002, 02:41
To add some more confusion: an Avro F/O once told me, that this plane is certified CAT IIIa because of the A/T. On the other hand an CRJ Captain said, that they were also certified CAT IIIa because of the HUD and despite the lack of an A/T. As far as I remember both Acro and CRJ have 2 A/Ps only.

[ 17 January 2002: Message edited by: xmp ]</p>

16th Jan 2002, 21:57
Probably worth noting that Cat IIIc does not exist yet, so there won't be any aircraft with approval for anything past IIIb.

Now, with 4 autopilots..... <img src="smile.gif" border="0">

17th Jan 2002, 09:32
S'right Scooby. Thats the definition of 'Fail Operational' and why you can perform a CatIII landing with a 'Fail Operational' AFCS but not with one that has degraded to 'Fail Passive'. A Boeing will tell you as far down as 1500 feet if one channel is voted out, by annunciating 'No Land 3' but after 1500 feet will keep that information to itself until below 80 knots on the roll out. I know of no AFCS with less than three channels that may perform a CatIII Approach and landing. Two channels and you're restricted to CatII at best - i.e. see the runway at minimums or b*gger off somewhere else. Of course 'channels' is not the same thing as 'Autopilots'. You might for instance say that a B767 has only one autopilot with three channels but really its a Triple Channel AFCS with integrated Autopilot, Flight Director and Autothrottle functions.

Through difficulties to the cinema

17th Jan 2002, 10:05
A310-300 & A300-600 = 2 AFCS systems each with a Command and Montitor channel. Ergo, Fail Operational/Fail Passive assured.
The purpose of multiple system:
1) To allow Cat 111 + autoland and redundancy
2) To give pilots something else to b*gger up
3) To give avionics engineers the ability defer stuff and still prevent the pilot from really flying the aircraft (lot safer that way).

17th Jan 2002, 11:15
The HUD CAT IIIa capability on a CRJ is strictly for manual flight only - the Autopilot (CAT II if you want to use it) is only used to get you to below 2000 ft agl when AIII mode is selected on the HGS. The pilot must disconnect & fly manually by 650ft agl or he gets an Approach Warning (= Go Around). Monitoring is carried out both inside the HGC and by the Co-Pilot checking pilot performance on his PFD. Again any failures or not keeping the aircraft within limits ("making donuts") and you'll get an Approach Warning & MUST do an immediate GA.

18th Jan 2002, 00:26
The B733 has, according to Boeing, only two autopilots on board but is certified up to CAT IIIa. So automatic approach and landing is possible with only two Autopilot-Channels. The system is specified as fail passive throughout approach, flare, touchdown or go-around. If one of the two channels fails, the other will counteract this channel and disconnect both autopilot-channels from the flight-controls.

18th Jan 2002, 20:56
Siddique, not only the Tristar has the dual dual config, also the MD-11. Two autopilots, Cat 3B capable. A great system, and incredibly reliable. As for the other comments posted earlier, we use the left or right A/P depending on which pilot is Pilot Flying (we haven't yet adopted Nigel's system of nomenclature so we don't have Landing but Not Handling Cockpit Members), strictly as a means to exercise both sides of the house. Each F/D is tied to its respective A/P, so system monitoring is always present by the pilots, and also by the comparator.