Landing and Take Off Minima

[cavelino rampante]

Why are landing visibilty minima (200m CAT III) higher than takeoff minima (125m)?

[Wizofoz]

Because they are based on different criteria.

Takeoff minima is based on the minimum vis considered needed to keep the aircraft straight in the event of an engine failure and/or RTO during the takeoff roll (what you quote is cat C minima, it's 150m for cat D).

CatIIIa minima is based on fail passive autoland, and is the minimun vis (plus DH of 50') considered needed to take over and land or go around in the event of an autopilot failure. CATIIIb is based on fail-operative, so either has a very low DH (e.g 15') or none at all, and very low vis requirements (50m).

[popay]

how about slant vision?:8

[Wizofoz]

Errrr..... How about slant vision?:confused: :confused:

[BigEndBob]

What height above the runway is visibility measured?

One would probably have a better view of the runway from a 747 flight deck than a c_152, as regards slant visibility through fog on t/o.

I was once told on three occasions approaching Birmingham in my C-152 at 3am in the morning the visibilty(RVR) was 50m, yet i replied visual with the runway.
Wasn't until i started to roundout that i hit a shallow bank of low mist that couldn't have been deeper than 6 feet hanging just above ruway.
No problem landing with the landing light off.

[Angels One Fife]

And you do know about an Approach Ban and RVR, right?

[popay]

Wizofoz, all you've said is correct. However the question is WHY take off 125 and CAT III a 200 in other words why RVR 200 is required for CAT IIIa and not RVR 125 like for the take off.
The answer is cause the law maker leaves a back door open for an unlikely event of a manual landing with fail passive shooting a CAT III a. In order to perform such a landing a min of 200 rvr is required (actually its 300 meters established throughout an empirical methods but may be reduced provided an aircraft with a fail-passive automatic landing system which is supplemented by a head-up display which does not qualify as a fail-operational system but which gives guidance which will enable the pilot to complete a landing in the event of a failure of the automatic landing system. In this case it is not necessary to make a go-around mandatory in the event of a failure of the automatic landing system when the RVR is less than 300 metres.) The figure 200 is because of the geometrical position of the pilots eyes which sees the runway's segment in a slant vision. In other words when the aircraft is on the runway the pilots only needs 125 m RVR in order to perform required action consequently when the a/c is at 50 ft with 4 deg pitch the pilot sees a segment of the Rwy in slant vision and therefore 200 RVR m is required in order to take over and land manually. 200>125 because the vision of the pilot at 50 ft is more obscured than on the ground. Perhaps the term slant vision is not appropriate it's obscured one.
Cheers.

[popay]

Hi found some more info on that one
Correlation between visual segment: SVR ; RVR
A formula can be used to compute the SVR required for the pilot to acquire visually the specified visual segment, at the DH for a particular operation.
v : visual segment (m)
h : pilot's eye height above ground level (m)
w: Cockpit cut-off angle (°) = Down vision angle (°) - pitch angle (°)
SVR = square root of ( v+(h * cotan w) )² + h² (purely geometrical relation)
Example of establishment of required RVR
For CAT II operations with auto pilot down to DH=100ft and manual control below,
the required visual segment is 90m. The required SVR for such operation is 220.7m
(using the example as shown in Figure 2.4).
Eye height = DH + 20ft =120ft = 36.6m
Cut-off angle = 20°
Pitch = 4°,
w = 16°,
Visual segment = 90m
Using above formula we find SVR =220.7 m
At 120ft above ground level, the SVR/RVR is expected to be 0.68 or more, so we obtain a required RVR equal 324.6m. RVR = SVR x 1/0.68 = 324.6 m (for SVR/RVR=0.68) The same method may also be used to evaluate the visual segment for a given RVR
Interesting observation is that the pilot sees the same segment of 90 m being at 100 ft as at the ground for LVTO. If one remembers, 90 m visual segment is required upon line up additionally to all the relevant RVR readings.
Cheers.:ok:

[BigEndBob]

Angels one five.

What had happened was that mist from small stream flowing through the airfield was producing a shallow bank of mist drifting through the touchdown area. The rest of the runway was clear, yet 50m was being reported.

Just interested to know where the RVR detectors are located, and why the controller just couldn't say that some mist was drifting across touchdown area, instead of saying 50m RVR when the runway could be seen from 10 miles out @ 3000'.
By the way the mist was 6 feet above runway about 2 foot deep.

[Tea_No_Sugar]

Popay

Repeat after me

" I must try harder to get out more"

[Jo90]

When I operated the 767 we had a TO minimum of 75m ( same as our CatIII NDH landing minimum ) because the a/c had a PVD (Para Visual Director ).

If you havn't seen one of these it looks like a barbers pole set horizontally in the edge of the glareshield straight ahead of each pilot. It is controlled by the flight director according to localiser deviation and appears to stream to left or right if you deviate from the centreline. If you lose sight of the centreline lights because of a thick patch of fog you simply steer the a/c in the usual manner so as to follow the streaming, ie to keep the stripes stationary.

Never had to make use of it in anger but it was pretty easy to use in the sim.

[Georgeablelovehowindia]

Jo90: The PVD, as featured on that earlier, pioneering aircraft, the Trident! (Not that I ever flew it, mark you. One jump-seat ride convinced me I'd never fathom that Smiths flight system.) :bored:

[singleseater]

Speaking for the 777
mins at heathrow for instance
takeoff 777-200 (cat c ) vis 125 m
landing Cat IIIb vis required 125 m DH 0

777-300 (cat d) vis 150 m
also 150 for Cat III b landing

1 or 2 engine makes no difference

vis segment for both is 90 m required. ie must be able to see, from the flight deck an unlit object, the size of a person at 90 m from the nose of the aircraft

However, these are reported v is from the RVR sensors.
What is defined as visual ref at min for a III b is any single light, sufficent to judge the lateral placment of the aircraft. There is no time to make an assessment of ability to keep the aircraft staight, just that it will be on the runway. Some of the vis requirement also to do with being able to get the aircraft off the runway safely. Ground radar is fine, but it can't turn the thing.

These are company mins, buteach operator has to be approved by the dept of the country in which the appr is taking place.

[Kak Klaxon]

I have just started work at a new company,in the sim on a low vis Take Off I started to count center line lights to check I had 90m visual segment,sim guy said that there was no need to do this,in fact no one at this company has ever done this,they just go if it s 125m,multi RVR.

Have I been doing it wrong for so many years?

[popay]

Kak Klaxon, hi there you did absolutely correct. Here is the reference:
Subject to the approval of the Authority, and provided the requirements in
paragraphs (A) to (E) below have been satisfied, an operator may reduce the takeoff minima to 125 m RVR (Category A, B and C aeroplanes) or 150 m RVR (Category D aeroplanes) when:
(A) Low Visibility Procedures
are in force;
(B) High intensity runway
centerline lights spaced 15 m or less
and high intensity edge lights spaced
60 m or less are in operation;
(C) Flight crew members have
satisfactorily completed training in a
Flight Simulator;
(D) A 90 m visual segment is
available from the cockpit at the start
of the take-off run; and
(E) The required RVR value
has been achieved for all of the
relevant RVR reporting points.
(ii) Subject to the approval of the
Authority, an operator of an aeroplane using
an approved lateral guidance system for
take-off may reduce the take-off minima to
an RVR less than 125 m (Category A, B and C aeroplanes) or 150 m (Category D aeroplanes) but not lower than 75 m provided runway protection and facilities equivalent to Category III landing operations are available.
There's also a note saying that The reported RVR/Visibility value representative of the initial part of the take-off run can be replaced by pilot
assessment. There's as usual different interpretations of that note. Some people are saying that it's only possible if the TDZ RVR isn't given. The others are saying it's irrelevant and regardless of the RVR given foe the initial part (even if it's reported below let's say min is 125 and reported TDZ RVR is 100) it can be replaced by visual means. Anyone got any idea?
Cheers.

[flyboyike]

Why are landing visibilty minima (200m CAT III) higher than takeoff minima (125m)?

Shouldn't it be cavallino rampante?

[desmo3]

There's also a note saying that The reported RVR/Visibility value representative of the initial part of the take-off run can be replaced by pilot
assessment. There's as usual different interpretations of that note. Some people are saying that it's only possible if the TDZ RVR isn't given. The others are saying it's irrelevant and regardless of the RVR given foe the initial part (even if it's reported below let's say min is 125 and reported TDZ RVR is 100) it can be replaced by visual means. Anyone got any idea?


hi there,
any idea where to find "the final" answer about it?
I'll give you a scenario.
cat c a/c; not airport restrictions;rvr all relevant.
rvr 100/150/150
can you disregard the first one in accordance with the discussed note, or you are in the 125/125/125 paragraph where "The required RVR value
has been achieved for all of the
relevant RVR reporting points."
My interpretation suggests we can go...but that's only my interpretation....
thanks for the help
Cheers

[alf5071h]

popay, the formula at #8 assumes a homogeneous fog, which does not consider stratification (fog layers during formation or shallow fog) or cumulo forms (fog ‘clouds’ when dispersing), in these conditions the horizontal visibility can vary with altitude (cf #5) and hence change the slant visual range (SVR). These are the more complicated (hazardous) fog situations where a decision can be made based on the required visual scene and then it reduces; they are more often encountered in Cat 2 conditions. Thus SVR is not a consistent parameter for determining the visual segment.

The complications caused by this were simplified by using RVR (measured at 15m altitude) and a predetermined visual segment (what the pilot requires to assess position / fly the aircraft) to provide a probability of approach and landing success; IIRC something like 98% prob of not loosing the visual requirement. This was based on the research at BLEU in the 1970s; however some of the more recent changes (JAA etc) reducing minima might not be as reliable. One such operation is ‘super fail passive’ Cat 3 (150m dual autoland) where the manual landing option when below 200m is removed to reduce the overall risk. Other changes may have omitted the effect of pilot's eye height.

Thus landing minima (RVR) is a function of the visual requirements to check position / land the aircraft, which can be a function of aircraft size and manoeuvrability (for manual landing).
Similarly take off minima are based on the visual requirements, but the task is different, to either conduct / continue the take off, or reject the take off. These requirements are also a function of aircraft type (ease of handling / equipment fit), which gives greater variability in the minima, and thus even more difference with the landing minima.

Visual assessment: when stationary at the threshold and knowing the runway / centreline lighting spacing, the RVR can be approximated by counting the number of lights seen – actually quite accurate.

[411A]

PVD came to corporate/business aviation as well, long ago.
The Gulfstream One was thus fitted...but only one that I know of, it belonged to the American radio/television personality, Arthur Godfrey.
CATIIIA was possible.

BEA TriStars were fitted with PVD, and it worked very well indeed.
Of course, it was not absolutely required, as the L1011 was CATIIIB right out of the factory, not added on (certificated) later.

Ahhh, Lockheed, a superb aeroplane...:E

[galaxy flyer]

The Lockheed (what else!) C-5 also had PVDs for both pilots mounted on the glareshield. It was evidently part of the installed autoland system, but the system including the PVD was not maintained and autolands to touchdown were not authorized. The PVDs worked and was good to mystify Lts until they were removed.

GF

[keithl]

So what were the Lts removed for?

Someone had to say it...