Maybe my simplistic view. Why can't you dispatch with two destination alternates as long as both of those satisfy the dispatch criteria?
If your dispatch calculation shows that you cannot land on the most favorable rwy in still winds but you can land on the rwy most likely to be assigned given the forcasted wind/weather conditions you can dispatch with two alternates that comply fully with the dispatch requirements. When doing this you need to perform a new calculation before landing with the latest weather reports available, and satisfy dispatch criteria on the runway assigned to you. If this calculation fail, even though your inflight assessment of a safe landing permits you to land, you are not legally allowed to land on that runway.
However; This exemption rule cannot be used if your destination have more than one runway.
If you can land on the most favorable rwy in still wind but not on the rwy most likely to be assigned given the forcasted weather conditions, you may dispatch with only one alternate complying fully with the dispatch requirements.
OPS 1.515 Landing — Dry runways
(a) An operator shall ensure that the landing mass of the aeroplane determined in accordance with OPS 1.475(a) for the estimated time of landing at the destination aerodrome and at any alternate aerodrome allows a full stop landing from 50 ft above the threshold:
1. For turbo-jet powered aeroplanes, within 60 % of the landing distance available; or 2. For turbo-propeller powered aeroplanes, within 70 % of the landing distance available;
(b) When showing compliance with subparagraph (a) above, an operator must take account of the following:
1. the altitude at the aerodrome; 2. not more than 50 % of the head-wind component or not less than 150 % of the tailwind component; and 3. the runway slope in the direction of landing if greater than +/-2 %.
(c) When showing compliance with subparagraph (a) above, it must be assumed that: 1. the aeroplane will land on the most favourable runway, in still air; and 2. the aeroplane will land on the runway most likely to be assigned considering the probable wind speed and direction and the ground handling characteristics of the aeroplane, and considering other conditions such as landing aids and terrain.
(d) If an operator is unable to comply with subparagraph (c)1 above for a destination aerodrome having a single runway where a landing depends upon a specified wind component, an aeroplane may be despatched if 2 alternate aerodromes are designated which permit full compliance with subparagraphs (a), (b) and (c). Before commencing an approach to land at the destination aerodrome the commander must satisfy himself/herself that a landing can be made in full compliance with OPS 1.510 and subparagraphs (a) and (b) above.
(e) If an operator is unable to comply with subparagraph (c)2 above for the destination aerodrome, the aeroplane may be despatched if an alternate aerodrome is designated which permits full compliance with subparagraphs (a), (b) and (c).
I think you've summed it up quite correctly. You've also added a couple of questions:
Is it unsafe to takeoff without safety margins for landing at destination?
It's not necessarily unsafe to do this as long as the conditions doesn't change enroute, but it's not legal. Definition of safe is not a regulatory issue. The regulatory agency has made the definition of safety up to national agencies/operators/commander to determine. For instance; one operator could get approval for one type of inflight assessment based on their training, type of equipment, experience level etc etc etc, whilst another operator would not.
Is it safe to land without safety margins in case the surface conditions at the time of landing are different from those planned at departure?
Not necessarily either. Not advicable perhaps, but not necessarily unsafe. Again see the answer above. Say your LDA is 7000 ft and your calculated unfactored landing distance is 7000 ft. If you fly your aircraft exactly how the unfactored landing distance was calculated. Vref+0 at 50 ft. Touchdown 305meters down the runway, application of maximum wheelbrakes 1 second after touchdown, deployment of reverse 2 seconds after touchdown up to full reverse down to 60 kts, before using wheelbrakes to come to a complete stop. Your nosewheel should be at the edge of the asphalt at the end of the runway. Certainly not unsafe. You can taxi fat and happy back to the gate, and all passengers are safe and sound without any injury to pax or damage to the aircraft. ARE MOST PILOTS OK WITH THAT ? Certainly not. How much margins you add are up to you, but I would certainly consider; Worn brakes, MEL, long days with fatigue, unreliable weather reports, unreliable friction measurements, worn tyres, increased ref speeds, pilots experience level, light conditions, and on and on and on. How much margins ? If using the dispatch margins you've given yourself a lot of margins. I would even say too much margins since you would have problems even landing at all applying all those factors, but you're not wrong to do so. It's all up to you. You can even add more margins if you deem it necessary.
If rules can specify precise factors for preflight planning then why cant they do that for inflight planning?
What do you mean by inflight planning ? Planning ends when the aircraft is moving under it's own power for the intention of flight. You do your planning on the ground before commencing your flight. If you receive information enroute to your destination that your alternate airport is closed due weather, you need to perform a replanning. A replanning must meet regulatory requirements, hence a dispatch calculation for your new alternate must be successful. Rules or rulemakers do not specify specific criterias for inflight assessments. They require you to have the methods/SOP you intend to use approved. That's why you sometimes see some operators land and some don't because their approved criterias are different.
9.G, “No brainer there, if you ask me”.(#17) A pre landing check / reassessment should be standard practice; this involves conscious thought about the inherent risks. If the example 7000ft landing distance was based on ‘actual’ tables, it perhaps overlooks the more representative 8600ft, consisting of +600ft realistic airborne distance, +500ft for 10kts fast on a wet runway, and +500ft for 2sec delay in braking, - all normal operational behaviour (AC91-79 data). The revised distance still requires an adequate level of braking, timely selection and use of reverse thrust, and assumes that the runway braking action and wind are as reported. Without these, where the latter depend on other people, a 9000ft LDA may only give a 5% margin, but with other considerations such as type of runway surface, or exactly how wet the runway might be; no safety margin. Think about it.
Airbus has introduced operational landing distances containing necessary margins and covering various runway conditions, aircraft status and failures.
The assumption was based on the new OLD concept. With the OLD one can also use the physical length of the RWY as sufficient margins, as mentioned by you, are built in. One can exaggerate and picture the end of the world or simply be aware of the assumptions and inherent risks, not excluding incompetence.
9.G, I agree that OLD provides a better basis for landing performance, but it is only a basis, which has to be evaluated against the current situation before landing. It’s not necessarily ‘incompetence’, but the normal variability of human performance, both in flying and judging the risks in a situation. There are good examples of these in the Boeing analysis - pages 8-13.
In addition to the three main items of aircraft control - touchdown distance, speed, and spoiler / brake application, there is combination of circumstances, such as tailwind and medium / poor braking (Table 1). Also note the distance margin used for the normal (non-long) touchdowns; up to 20-30% of the available distance.
An operator shall ensure that the approved performance data contained in the aeroplane flight manual is used to determine compliance with the requirements of the appropriate subpart
Folks, The landing field length from the AFM will be certified data, ie; it has been factored. This is the landing field length for all but emergency operations. Basing any of your operation on "raw" test flight numbers has no place in any commercial operation. Likewise, "tricky" interpretations of rules by bush lawyers has no place. It is highly unlikely that the average pilot, in the average aeroplane, would even get close to the raw (unfactored) field lengths achieved during certification test flying. Assuming the above quote is correct, I would have thought that would settle the argument. Tootle pip!!
PS: FokkerJock, Is that intended to be a definition of how test flight landing field lengths are established --- it's a new one on me.
No it's not. It is the conditions upon which the advisory performance information in the AFM are based on. For complete definitions of how flight testing data are validated for aircrafts certified under FAA regulations you need to look up FAA AC 25-7A "flight test guide for certification of transport category aircrafts" or EASA CS-25 for the european norms.
I can only assume that the conditions listed in the AFM are derived from one of the three AMC (Acceptable Means of Compliance) listed in AC 25-7A for determination of Airborne distance.
I was not present during the first B737 certification so this is what I was able to dig up on the subject. In fact I wasn't even born
Last edited by Fokker-Jock; 28th Dec 2012 at 14:36.