View Full Version : CAT II app -737

21st Aug 2004, 13:52
Are you authorised to do a manually flown CAT II app on the 737 ?
I'm asking you cause I was surprised to learn that we are not authorised to perform a manually flown CAT II app on the 737.
The AFM for the NG says that we can do a dual chanel app with manual landing,but our SOP it's more restrictive:only autoland !
So,with all the FDs,GPS,etc ,if an autopilot fails for some reason you won't be able to do a FD manual CAT II app?
Why? What is your practice regarding this?
On other,older planes I flew, we used to do manual cat II app,no problem.
Brgds Alex

Notso Fantastic
21st Aug 2004, 14:59
A CAT 2 is with dual A/P. A CAT 1 is manual. If you were permitted to do a manual CAT 2 then what is the point of having CAT 1 approaches at all?

21st Aug 2004, 16:18
Nice one ...:D
: 2 chanel -cat 2 ,
single ch-manual -cat 1 .....:p For cat 3 of course you'll need 3 autopilots :cool:
I think you're joking,of course.
Didn't you ever fly a manual cat 2? Not all the a/c are capable of autoland,you know?!
So,if you're on an autoland capable a/c,and one of you're autopilots is u/s ,you won't be allowed to perform a cat 2 app?
I'm interested in 737 operators also.
NSF ,all respect for your 744,beautifull a/c,but I think this is not the first type you're flying,am I right?All you flew with, did autoland? Lucky you!
Brgds Alex

21st Aug 2004, 18:07
In our B737 FCOM (AOM) Vol.1 (Limitations L.10.6) it is stated that (and this is for NG):

The airplane is approved for the following types of ILS approach and landing:
DH below 200 feet but not less than 100 feet (Category II) (Flaps 30 and 40).
- Dual channel automatic approach and manual landing.
- Dual channel automatic approach and landing.

21st Aug 2004, 20:50
Hi Folks

Ill give you the lowdown on Autoland.

The Categories refer to the operational requirements not directly to the aircraft autopilot.

CAT 1 = This is an IFR approach with a Min Decent Altitude of no less than 200ft. and a Runway Visual range of 1800ft. This is a manual Landing.

CAT 2 = This is a precision approach with a DH=100ft and a RVR=1200ft.

CAT 3 is split into 3

3a is a Precision Approach with no DH minima and RVR not less than 700ft. The aircraft must have Fail passive or Fail operational Autopilot.

3b is a Precision Approach with RVR less than 200ft but not less than 150ft and a DH of 50ft or less. Aircraft must have a Fail operational autopilot system.

3c Is designed for Zero-Zero approaches no RVR limit and no DH limit. 3c operations are currently not authorized!

Now as for the aircraft,

Fail Passive, this means if one Autopilot channel or one or more sensors to the A/P (EG Radio Alt) fails the A/P will continue the approach safely with out massive deviation, but the landing would have to be carried out manually.

Fail operational means where one channel or input fails the aicraft can continue to Autoland, but it now becomes fail passive. A futher subsequent failure will mean a manual land.

The 747-400 i work on has a fail operational autopilot system, it has 3 autopilots and the inputs are all triplicate too. with everything ok all 3 autopilots engauged on the approach it is said to be fail operational, CAT 3b, if somthing were to fail it would downgrade to land 2 and become fail passive max CAT3a.

As for the 737 it has 2 autopilots this means its fail passive and will be CAT3a

BA 737-400 CAT 3 Placard on flight deck


21st Aug 2004, 23:05
Functioned Satis,

You're slightly behind the latest developments. Using a Head Up Display you can be certified for a manual catIIIa landing.

Have fun;)

21st Aug 2004, 23:24
alexban - I understand there is some 'unease' in airlines at the moment about the practice of manual landing in low vis without the extra sim practice for handling pilots. I believe it is to do with the false visual picture you get in those conditions and judging the flare, but I could be wrong.

I guess with single autopilot you will need that CAT I alternate:D

21st Aug 2004, 23:48

I was'nt aware of that.But my post was talking about Automatic landings the 737 can do a Automatic CAT 3a without a coupled rollout. what i was trying to help the origional poster with, was that the Aircraft and operational categories can get confused with each other.

Im an engineer so ops is not my field you'll understand.


22nd Aug 2004, 09:53
The FD bars will retract at 50 ' so you won't be able to do a cat 3 manual app.But BIZ is right,the head up display installed on the 737 or Saab 2000 is certified for manual cat 3a operations.
But this was not the case for my initial question.
Satis ; the ng can be passive or fail operational,depending on configuration demanded by the operator.
And i think I can assure you that no profesional pilot will confuse a/c type with app type.
Noac :indeed .one problem can be at DH,when you lift your eyes from the instruments to look outside,you can have some dificulties to accomodate .

22nd Aug 2004, 10:48
Are you authorised to do a manually flown CAT II app on the 737 ?

Hi Alex, we are not. Automatic cat II only, manual landing.
But to my knowledge this is only an SOP restriction (as usual they don't trust their pilots...), not a Boeing limitation.

Second channel engagement is not required for cat II, but we usually engage it anyways and set up for a cat III autoland.

22nd Aug 2004, 15:17

" And i think I can assure you that no profesional pilot will confuse a/c type with app type"

I do not doubt that whatsoever! that was not the meaning behind what i said, ive seen it said not in jest that CAT3 = 3 autopilots etc thats what i mean. I do sometimes hate posting in forums becuse of the whole missunderstanding thing:\ why do threads often go this way? I try to help, i do not claim to be the total authority on anything, far from it.

Id like to find out more on how the ng could be fail operational. Ill look into that one if i can or someone here may find the answer:ok: I work on 3 autopilot aircraft so 737 i have no knowledge of.


Stu :ok:

22nd Aug 2004, 19:14
A Cat 2 manual approach (flight directed) requires the aircraft to be certificated for the operation and the clearance and limits be published in the AFM. More recently for aircraft types that have autoland, the manufacturers are reluctant to spend additional time and money on achieving a Cat 2 manual certification. The tests require proof that the FD guidance and pilot technical error are satisfactory; this is similar to the certification required for auto approach and HUD (Ref JAR-AWO).

The RVR limits for manual landings in low visibility are conservative and should not require extensive additional training. One good simulator period should give crew sufficient confidence and demonstrate that the general principles for landing still apply.

Cat 2 minima were historically 100 ft, 400m RVR, but with increased accuracy of flight guidance systems this has been reduced to 350m. Some operators have 300m, but this is normally associated with an auto coupled approach to 80% of the DH. Who has procedures for an autopilot failure between 100 ft and 80 ft? Are you allowed to continue or do you GA?

Research in the UK by the Blind Landing Experimental Unit (BLEU) in the late 70’s showed that it was feasible to land from 50ft in 200m RVR and from 35 ft
in 150m RVR; most operational limits are based on these data (old refs, ECAC Doc 17 and CAA computer model of landing in fog).

Thick fogs associated with Cat 3 RVR < 300m are generally stable with little variability. Thus where a pilot sees sufficient cues for a land decision then they should remain in sight and the visual scene generally improves as altitude decreases. Cat 2 conditions are much more variable and are often caused by adverse weather as well as fog. Cat 2 fog generally occurs during the fog formation or dispersal and thus is variable. A Cat 2 decision based on a visual scene does not necessarily mean that the conditions will improve below DH, thus crew should be aware that a GA below DH may be required. i.e. You can decide to land in Cat 2, but your decision could be mistaken or have to be reversed.

When flying Cat 2 HUD approach, who actually dispenses with the flare guidance and lands on the visual scene? How do you judge if the conditions deteriorate?

22nd Aug 2004, 23:56
As a policy we autoland if the weather is below cat I limits treating all approaches in these conditions as a cat III. This was the basis upon which our AWOPS approval was issued, and to change it to have cat II and a man landing would require further sim training to bring all crews up to speed.

Taking out the autopilots late on a dual channel approach to conduct a man landing will have the aircraft trimmed nose up for the flare and you risk destabilising the approach very close to the ground if you adopt that as a method.

Why complicate things the autopilots do a fine job and if we want to hand fly down to cat II limits I am sure that the TRE would let us have a gash few minutes in the box:ok:

Cap 56
26th Aug 2004, 16:01
A cat II with manual landing is allowed if you keep the autopilot engaged till 80 % of the minima.

This would for example allow you to land at an airport with cat II weather and a loss of system A.

Of course your flying skills have to be up to speed.

However breaking clouds at CAT I minima with a cloud deck at 200 ft in rain and a strong cross-wind would be more chalanging.

26th Aug 2004, 17:08
Cap 56 depending on the type of system in your aircraft I would carefully check the small print in the AFM as you may not be allowed to approach and land at an airport with cat II weather after the loss of system A. The certification rules may require that both channels of your system to be working for a coupled descent below Cat 1 minima.

Why should you expect a manual landing in Cat 2 to be as demanding, or more so, than a normal approach and landing. The assumptions in aircraft and operational certification are that any licensed crew will be sufficiently will trained, nor require exceptional skill, etc, to land an aircraft within the limits laid down.

27th Aug 2004, 02:00
In the USA it depends on your Ops. Specs. At our carrier, we are authorized hand flown Cat II approaches to 100'. The autopilot is NOT required.

Of course, we train and check to those minimums (with the most critical engine inoperative).

30th Aug 2004, 02:02
It would make sense to me to be able to fly a single channel CAT 2 approach with a manual landing if for some technical reason, A/P U/S etc a dual channel approach could not be flown. Single Channel approaches do not produce the nose up trim of the dual approaches on the final stage of the approach,so do alleviate that problem.

The J-41 was certified for CAT 2 approaches and only had one autopilot..what is the difference really? Well apart from size, speed and noise!!!!

I should imagine it is a company thing, it would take extra time in the sim ( they can be a tad tricky!) .And I can understand that during the low vis ops time in the box, they would prefer us practising CAT 3 approaches.

If i remember correctly, and I will have a look in the books tomorrow, the early generation 73s are fail passive and the NGs are fail operational but for commonality companys treat them as the same.

30th Aug 2004, 12:26
quid, jetjocky737 I refer you to my previous post; before you or the local inspector can authorise an operation the certification standard of the system or what part remains serviceable, has to be understood.

I have experience where a Cat3 HUD operator was given local authorisation for take-off in lower RVR than originally certificated. What the operator and local inspector were unaware of was the lack of an adequate HUD control law to provide guidance for an engine failure; the original low vis take-off certification had assumed that there was some external visual scene, thus an engine failure at the reduced RVR value would have been hazardous.
For 73 vs NG, check the details; Cat 3a vs Cat 3a enhanced (lower RVR?), procedures after failure, and crosswind limits; I hope that the safest common denominator is used and not the best operational capability.

For the J41 or any other system, they all require a method of cross monitoring sensor inputs and autopilot (F/D) output. In a dual autopilot the ‘second’ system or channel usually provides the monitor. Single channel Cat 2 autopilots and FDs usually have an additional monitor computer; they light the Cat 2 ‘green light’ when operative.

Therefore, it is incorrect to immediately assume that the operational capability of a system can be applied to different scenarios or that a degraded system’s approval extends to alternative approach procedures; don’t assume – check.

Cat 3 sim training; why do so many regulators / operators spend so much time checking standard words / procedures (normal operations), when the critical issues are the failure cases – continue to land after a failure or GA. For most Cat 3 operations (not withstanding my earlier post on Cat 2 problems), there is little point in checking rechecking the visual contact and decision making in the sim. The sim rarely represents the more difficult visual cases accurately, and in real operations, providing the operation is conducted within authorised limits, the crew will always see sufficient visual cues to land (90% plus combined probability of everything working – ILS, autos, and visual segment). Modern systems are very reliable; crews under stress may not be so good.

30th Aug 2004, 21:11

Well worded explanation!! I would never assume that a system that was degraded would be appropriate for use until I told by the company. I was just saying that I could see no reason why a CAT2 approach couldnt be flown with one u/s autopilot, as there is no glaringly apparent reason...until it is pointed out by somebody else!

Not at home to check the 73-NG differences but I will keep you posted!

1st Sep 2004, 00:16
Jet jocky

Your right there is no reason,on a 2 autopilot system if one A/P is u/s or an input to it. the Eng would downgrade the aircarft from CAT3 to CAT2.

There's a few permutations becuse Autoland is treated slighlty seperate to Autopilot by engineerig, you could have 2 sevicable auto pilots but both Rad Alts are stuffed, that would mean downgrade to Manual land. same for the ILS recvs and IRS.
As safetepee says you cant assume with all aircraft how system servicability is determined it can get quite complicated, thank god for the MEL!!!



Cap 56
1st Sep 2004, 12:48
My previous post was not accurate since I was not able to check the B 737 NG AFM.

Basically the concept of CAT II and III has all to do with the definitions of “Fail Passive” and “Fail active” autopilot systems.

According to the B 737 NG AFM one needs both Autopilots to do a cat II and this implies that an automatic landing is the only option.

If a technical failure arises AFTER you have passed the approach ban then you have to abort the approach.

On the B 737 EFIS a cat II single channel was allowed and I do not see why it has changed on the B 737 NG.

Maybe the increased risk of tail strike has something to do with this ???

Anyone can shed some light on this ?

1st Sep 2004, 17:15
cap56 :that is why I've started the topic. On the 737 Clasic also,single chanel Cat 2 is not allowed.Only dual chanel with manual landing or autoland.
Cat 1 app is allowed with SCh,Dch or FD manual app.
I've found all this restrictions written in the AFM,so it is established by Boeing,not by the SOP.

1st Sep 2004, 19:32
The essential elements required for an approach system are much more than the auto flight computer, sensors, etc. I recall that there are electrical distribution differences between 73 and NG aircraft. The changes were essential to get approval for a fail operational system.

Also beware the dual Rad Alt failure – revert to Cat 1. Some aircraft use Rad Alt to schedule gain changes in the FD as well as the autopilot, thus although the aircraft is Cat 1 capable the FD performance may not be very good.

Cap 56
1st Sep 2004, 20:19

On the B 737 EFIS a cat I with manual landing was allowed if Autopilot was not disconnected before 80% of the DH.

Basically Boeing is not responsivble to set the limits. Boeing has to prove under what conditions the aircraft is fail passive or fail operational.

The certifing authority then approves the capability.

1st Sep 2004, 22:33
Just to answer a previous query raised about Cat IIIb on the 737, it has been available for about a year.

To be honest I don’t know what the physical differences to the aircraft are but on the flightdeck you will see an extra MFD button labelled “C/R” (Clear / Recall), this must be pressed before every Cat IIIb approach to check the systems. The result will be a message on the upper DU which will either say “RECALL” which is OK or "NO LAND 3" or "NO AUTOLAND". Note Cat IIIa is still possible with a NO LAND 3 advisory. In this case green "LAND 2" annunciations will appear on both outboard display units. When established on the Cat IIIb approach you will get a ROLLOUT armed annunciation underneath VOR/LOC in the same way that you get FLARE armed under G/S for a Cat IIIa. More details at
www.b737.org.uk/flightinsts.htm (http://www.b737.org.uk/flightinsts.htm)

Note, aircraft with this mod are able to autoland with one engine out – v. impressive!

Cat II in a 737

To answer the question of the thread; all 737’s are certified for Cat II manual landings but it is up to the individual operator to approve and train their crews to do this. None of the 737 operators I have flown with have chosen to do this and to be honest I am grateful.

S & L

2nd Sep 2004, 00:36
Fail operational = the initial condition of a 3 autopilot system, a failure would mean the autoland is still OPERATIONAL. LAND 2

Fail Passive = A second failure of a 3 a/p system would leave only one A/P resulting in a fail passive meaning a subsequent fail would leave the aircarft in a passive condition.

my expirience is of only 3 chanel systems, i do belive however that the 737 is capable of fail operational with some magic it does!

Cap 56
2nd Sep 2004, 09:40

You are wrong.

CAT III requires fail operational and we have had this for years on aircraft with two autopilots.

2nd Sep 2004, 11:19
CAP 56

CAT 3 is separated,

CAT 3a requires fail passive or fail operational autopilot.

CAT 3b Requires fail operational autopilot.

I am aware that the 737 can carry out CAT 3a, allthought with what ive learned, it is fail operational so it could do 3b. Im not sure how but im trying to find out at work how a 2 autopilot system can be fail operational.

safteypee i think got it when he mentioned the bus isolation on the 737. The 747 and 777 does this too. splits the buss supply to the AFDS into independant supplys, the 737 does this too and thats partly how it can achive fail operational 3b.

This is an intresting thread where im learning too, more on the operational side of autoland. im a licenced avionic eng and i like to know how the ops side of things are as well as the tech.

i hate the way ppl get so aggressive in forums you said to me "Your wrong", a very strong statment. pls read your post again. please be a bit more courteous no one knows it all. Any who claim too are fools and id be scared if they were pilot or engineer!


2nd Sep 2004, 20:13
Cap 56 life is not that easy; “Basically Boeing is not responsible to set the limits. Boeing has to prove under what conditions the aircraft is fail passive or fail operational. The certifying authority then approves the capability.”

The manufacturer has to determine the accuracy and reliability of the auto flight system to enable low vis operations. In addition the autopilot minimum use height has to be assessed (often a function of nose down failures, hence one use of trim up); in some aircraft the min use ht does not enable the lowest DH if the 80% rule is used, which for JAA operators only applies to Cat 2 and not Cat 1. (Appendix 1 to JAR-OPS 1.430). The autopilot min use ht should be published in the AFM and for FAA based AFMs there is usually a diagram showing the failure case recovery trajectory – check the assumptions made about this for your aircraft and for the recovery maneuver (1.3 g pull up?). Thus the Manufacturers limit may supercede the regulatory limit, and with a higher DH the RVR minima are also higher.

FunctionedSatis there is enormous scope for confusion between certification requirements and operating requirements; the generally accepted ICAO definitions were replaced some time ago by muddled thinking, mainly by the operational regulators (JAR-OPS).

JAR-AWO (equipment certification) is one of the better regulatory documents, it divides the categories by DH (100 / 50 / <50 ft), or no DH, and any rollout guidance requirements.

The RVR minima (visual cues) are operational limits; these are in a messy section (E) of JAR-OPS. However, with improving technology the boundaries of the minima are being eroded. i.e A ‘Super Fail Passive’ Cat 3 autopilot (still a fail passive system) may operate in 150 m RVR, which is considered Cat 3b visibility territory (Avro RJ), but the DH remains at 50 ft. Whereas Cat 3a 50 ft / 200-300 RVR allows a manual landing following failure, operations in 150 RVR mandate a GA after the failure.

The best advice is to read the manual, check the MMEL, and don’t assume anything.

Captain Stable
2nd Sep 2004, 21:15
FunctionedSatis - I note your comments about Cap56's comments and, like you, I found them on first reading aggressive and rude.

I am minded to issue a warning to posters yet again about the tenor of comments, questions and answers posted here. If you can't keep it polite and professional, then please don't bother to post. Go somewhere that adolescent behaviour is appreciated.

Cap56, I will leave off a personal warning to you as I am not aware of previous history on your part, and I realise that English may not be your first language.

{edit} On second thoughts, after seeing his contribution to the Balanced Field thread, Cap56 is banned from this forum.

3rd Sep 2004, 00:34
I have some information.

From the Boeing 737-300/400/500 Maintenance manual chapter 22 Autoflight.

(Operator BA)

" The Autoland system installed on BAB 737 is cleared for CAT 3A operation with the folowing limits:- DH=50ft RVR=200meters."

This would indicate to me that the 737 is a fail passive system.

As for the next generation types i have no idea an no access to any information.

Safteypee, thanks for that, very intresting i might try to get a read at JAR OPS JAR AWO.