Very short wet runway and thrust reverser efficiency

[Tee Emm]

I thought I would throw this open for discussion after a recent sim session on short wet runway operations. Assuming the approach is spot on and touch down occurs at desired point on the runway. The Boeing 737 FCTM states: "After the flare is initiated, smoothly retard the thrust levers to idle, and make small pitch adjustments to maintain the desired rate of descent to the runway. Ideally, main gear touchdown should occur simultaneously with thrust levers reaching idle".

It is well known that the thrust reversers are most efficient at high speed and for that reason they should be deployed as soon as possible after touchdown.

If the aircraft touches down with significant thrust still applied (sometimes happens inadvertently when there is failure to flare resulting in the classic "firm" touchdown which most of us have experienced in our career) - then the thrust levers N1 will have been probably around 55%-58% because the pilot did not have time to close them until too late. But there is one un-advertised advantage to that. If the thrust levers are closed sharply after initial touch down, and reverse immediately applied, the engines will already have been spooled up to relatively high thrust and will take less time to spool up to maximum reverse thrust.

On the other hand, according to the FCTM the thrust levers will closed as touchdown occurs. Now when reverse is applied the engine has to spool up from a much lower N1 - probably around 35-40 percent N1. That in turn causes a delay in reaching full reverse - perhaps several seconds and thus less efficient.

The rate of thrust spool up to max reverse becomes a significant factor when landing on a wet runway where braking efficiency is degraded somewhat. Putting aside for a moment the Boeing advice on thrust lever closure at the flare (and subsequent running down of N1 as this occurs) - would it be better to deliberately avoid closing the thrust levers until touch down occurs, then rapidly close them against the idle stop in order to permit operation the reverse thrust levers. In the simulator this technique has shown to allow a much faster wind up to full reverse because of the already high N1 at reverse deployment.

But - and there is always a BUT - this technique of touching down with thrust still applied needs to be carefully considered first, because it can lead to the problems discussed in another part of the FCTM and that includes a bounced landing which can occur because higher than idle power is maintained through initial touchdown.

Remember the scenario is where the runway is awfully short and wet and perhaps an alternate is not an option.

[flyburg]

personally, I would stick with manufacturers recommendations. The performance data is based on that.

Also, what you say, landing with thrust still applied, I my experience this usually leads to a prolonged flare or higher speed on touchdown. This being much more detrimental than the spool up time for the reversers.

Just my personal observation though.

Greetings

[CommandB]

I think im correct in saying that the performance for the 738 is based on the reversers being at the "interlock" ie- idle - no more than 2 seconds after main gear touchdown. And with use of 2nd detent (4secs to achieve) so if your LDR is less than LDA when looking at the QRH figures, then you should be able to stop using the FCTM landing technique. At least if something happens you can say you followed all procedures otherwise could make for a tea and biscuts moment...and bring your own biscuits! :}

[Mach E Avelli]

The most effective retardent is weight on wheels and maximum braking. Reverse is a bonus, so is obviously important on a short wet runway, but not the primary stopper. If it takes 2 or 3 seconds to spool up in reverse, not a big deal, as long as the lift has been dumped and brakes are being applied (maximum, if it's really that critical).
To ensure weight goes on wheels, the 'speedbrake up' action/call is the important one. Other gratuitous calls do exist - stuff like 'reverser unlocked' but I question the use of this extra verbiage. Gratuitious, because if reversers don't unlock, what then? If the spoilers are up, you are probably committed to stopping regardless. On the Bae 146, which has no reverse but is capable of near STOL performance, the important action/call is 'spoilers yellow/green'.
As for technique, I have never had a problem with coming back to Vref by 50 feet and snapping the thrust levers to the idle stops. Sometimes the flare needs to be a bit aggressive to catch it, but mostly not, because 1.3 Vso provides enough initial elevator authority at all but very low weight Vref. I can honestly say that I only ever did one really firm landing with this technique, and that was after an all-nighter onto a short strip.
On the other hand, I have witnessed some interesting bounces and long landings because pilots tried to use turboprop technique and were hesitant to close the thrust levers. Also, after a real 'greaser' onto a wet runway the speedbrake/spoilers don't always pop up on cue and have to be pulled manually.
However, I have no experience of the B737-800/900 (which I presume TeeM is referring to with the FCTM) and can understand why Boeing are now saying to close the thrust levers after initiating the flare (tailstrike?).
Some (not all) turboprops will fall out of the sky if full idle is selected too soon and the props are allowed to go below the zero thrust position, so these tend to encourage a trickle of power reducing slowly through the flare. Also, no doubt some of the stretched turboprops like the ATR-72 and Q-400 are at risk of tailstrike if an aggressive flare is made .
Turboprop exceptions that I know (there may be others) are those types powered by RR Darts which respond better to the idle stops at 50ft treatment. Then after touchdown and selection of ground fine, for maximum discing effect the power needs to be increased again to just below the point where the prop moves off the fine pitch stops. It also allows engines to run cooler. Even the longer Viscount 800 won't get to tailstrike unless the pilot tries very hard. Those old vibrators may have been complex but at least the pilot was rewarded with ease of handling.
I suppose the point of all this is that different types demand different techniques, but the runway behind you is still one of the three useless items in aviation (the others being fuel in the bowser and the sky above you).

[PENKO]

First, reverse is not the major braking source, that honour goes to the wheels and their brakes, even on wet runways. Second, it takes time for the reversers to deploy.

So with your technique you land with lots of additional thrust which will act against the wheelbrakes for as long as it takes for the reversers to deploy. Not sure if that is beneficial. Interesting thought though..

Anyway, on an Airbus you have no choice but to go through the idle stop before pulling reverse. Problem solved :ok:

[BOAC]

Interesting theory,TM - difficult as it may be, have you compared distances in the sim?

For the last 2 posters
1) The 737 LDR is specified with 2 x No 2 detent reverse. If no reverse there is an additive and it could be quite large on a wet runway. It is not 'a bonus'.
2) Boeing also requires idle stop before reverse levers can be moved.

[Cough]

Leaving thrust on until touchdown is probably going to mean that you land fast and long. Not good on a wet runway. Stick to the FCTM...

[Mad (Flt) Scientist]

On any a/c where the ground spoilers autodeploy on touchdown, it's likely that some combination of engine power or throttle position is part of the autodeploy logic. In which case, a higher than designed power level may inhibit autospoiler deployment, and as has been mentioned above, getting weight onto the wheels to let the brakes do their thing is of prime importance.

With the caveat that this may not apply to the specific type in question of course.

[johns7022]

I can't speak for Boeing aircraft, but in the planes I fly, TRs are not considered into the stopping distance data...

Nice to have though, especially on icy runways, can help with directional control and such...

As far as landing technique..I appreciate the 737 reference....makes sense...rather then your typical pilot chopping power over the fence, a smooth roll off of thrust to commence with a flare seems like a good technique.

Because I perform so many short field landing in turbojets, I prefer an early establishment of an approach speed without power....

[PENKO]

BOAC, I did not imply that reversers are not included in the LDR, just that their part in stopping is less than the wheel brakes.

[safetypee]

For wet runways, credit for thrust reverse is not generally included in certificated landing performance unless the manufacturer has a compiled with the demanding reliability requirements; CS / FAR 25.125.
For aircraft certificated under CS 25, where the manufacturer elects to publish data for contaminated runways, thrust reverse may be included (check the AFM small print). Thus, for these operations, in addition to having a very small safety factor (1.15 or effectively less), the actual landing distance depends on the crew’s consistency (technique) in selecting reverse, and a reliable operation of the reverser system; CS 25, AMC 25.1591. (Remember the problems at Midway – 737 fatal accident)

A graphical depiction of the relative effectiveness of retarding devices is in Fig 3. (http://flightsafety.org/files/alar_bn8-4-braking.pdf)

For a more in-depth view of the problems when stopping on low friction runways, see the presentations at a recent industry conference which is considering improvements, particularly for runway condition reporting.
IMHO several of the views raise serous doubts about the sufficiency of current landing performance when operating in adverse conditions.
Note the Airbus and Boeing views which I interpret as supporting regulatory change, i.e. longer distances.

EASA - Workshop Runway Friction and Aircraft Braking (http://easa.europa.eu/events/events.php?startdate=11-03-2010&page=Workshop_Runway_Friction_and_Aircraft_Braking) see ‘Presentations’, zip file for download.

[BOAC]

Safetypee - from a pilot's point of view rather than a theoretician's, the 737 requires 2 x No2 detent reverse for ALL surfaces for the reference LDRs in the QRP.

Penko - the 'bonus' was from the previous poster.

[safetypee]

BOAC, Re the 737 requires 2 x No2 detent reverse for ALL surfaces for the reference LDRs in the QRP.

Thanks for the info, but my understanding is that this relates to unfactored landing data which differs from the certificated performance in the AFM. It would be interesting to understand why Boeing presented it this way.
I would like to see the ‘conditions and assumptions’ in the 737 AFM for certificated (factored) landing data; I suspect they are as FAR 25.125 (with or without grandfather rights).

Another aspect is that the QRP assumes a fixed airborne distance from the threshold to touchdown, which I do not believe is the same for AFM data.
Thus, a problem of using the QRP data (planning, emergency, or before landing revisions) is that even after applying a distance factor, the actual distance required in the real conditions (actual safety margins) is not the same as those in the AFM.
I recall an example with the minimum 1.15 factor, which with operational variability in air distance, speed and height over threshold (also includes selecting rev detent), only equates to the absolute minimum landing distance achievable, i.e. there is no practical distance margin.

Also, I assume that you are fully aware, the resultant safety margin in a factored landing varies with the runway condition (how ‘wet’ is wet), the type of runway surface, and tyre condition amongst other issues. In extreme circumstances such as aquaplaning, it may be possible for the actual landing distance, including using reverse, to exceed the fully factored wet landing distance.
I suspect that many pilots, either consciously or otherwise (bias, hearsay, false knowledge), rely on reverse in marginal conditions. This is potentially hazardous behaviour, particularly on a short* wet runway; this might be a contributor to the large number of runway overruns.

* 'Short' in this context is not just the actual runway length, but it is the distance required, i.e. the field length limit involving all runway conditions / weight.

[rigpiggy]

wet/slick runway, hammer it on, get the wheels spun up for antiskid, get on the reversers straight away. look up cranbrook crash i think

[BOAC]

The point I am making, SP, is that as a pilot, when I am faced with deciding how I am going to land on a 'short wet runway', I have to take into account the figures in the QRP. I do not have the AFM, Boeing's Certification to whatever regs etc etc, and THEY are predicated on 2x Rev. It matters not a jot how the a/c was 'certified'.

As I have said many times before, the mantra heard often here about reverse being a 'bonus' or 'ineffective' will bite peoples' backsides one day with marginal braking action. Got it - use it - and use it properly!

[lederhosen]

Anyone in any doubt about what BOAC is saying should look at the report for the Southwest accident at Chicago Midway where they went off the end on a slippery runway.

[Cough]

Thanks for the info, but my understanding is that this relates to unfactored landing data which differs from the certificated performance in the AFM.

My understanding is for certified perf on a dry runway, the stopping perf assumes no reverse, but includes it on a wet runway. Hence BOAC's comment is correct.

[BOAC]

As always, Cough - thanks for the support (I am still wearing it...). A while back Mr Boeing snuck in a change to the 737QRP to have reverse on all LDR tables, including dry runway.

[safetypee]

BOAC, I understand your point. It’s a serious issue; the regulators often assume that pilots understand the limitations of various data packages and will adjust their choices accordingly. However, there is very little advice or guidance provided in these matters.

My last operation provided fully factored tables against landing weight for each runway, and these included wind and runway condition adjustment. For emergency use there was a standardised table for weight / altitude etc, from which the pilot could subtract factored distance according to the perceived risk. The subtraction method appeared to provide pilots with improved awareness of the distance remaining (the safety margin and conversely the increased risk), than the additive system required for the Boeing data.

IMHO an operational solution for this problem is to take a more strategic view of the situation, i.e. plan ahead; avoid rushing and the potential for biased assessment in a ‘tactical’ decision before landing.
The originating question cites a demanding situation, which several posts have considered as a ‘how can we do this’ situation. With appropriate knowledge of the risks involved when landing on wet or contaminated runways, then a better evaluation is to consider ‘should we be doing this?’

This type of thinking - decision making, helps to provide a safety buffer, perhaps assumed by regulation for these conditions. If you need to consider adjusting procedures to the point of inventing reason why a change is better, then perhaps that defines the boundary of a realistic safe operation.

Cough, take care not to confuse certificated data with advisory data. In some circumstances reverse could be used for certificated data, but I have not identified an example for landing. The use of reverse for wet RTOs if a different matter.

[BOAC]

We have strayed (as usual) from the OP asking whether 'tis better to have more N1 at touchdown to gain quicker reverse application - I still do not have an opinion on this but am interested in a refreshing new look at things.

Straying again, SP, I often find discussion about 'certification' and the like here, when I am looking, as a pilot, at what info do I have in my cockpit when my 'short but dry' runway with excellent LDA suddenly floods in a TS - can I or can't I? It is down to QRP, isn't it?

[safetypee]

BOAC, I am totally on-side with the need for a practical approach, but I don’t believe that ‘discarding’ certificated data - preferring a simpler alternative, meets the safety objectives specified by the regulators, or as required by our industry. Thus, due to the lack of simple guidance material, crews need to understand the basis of performance certification, and the application and limitations of the data.

The thread and many posts indicate a heightened level of concern about landing on a short wet runway; why?
Aircraft certification is the basis of a safe operation which requires a high probability (not certainty) that when using factored landing data (and the assumptions therein) an aircraft will be able to land on a short, wet runway without use of thrust reverse.
Using reverse reduces the residual risks in landing and may mitigate some errors, particularly those arising from unknowns associated with very wet or contaminated conditions. Is the routine use of reverse hiding these risks and thus crews are more complacent?

The professionalism of our industry requires that pilots have both appropriate information available to them to determine a safety landing distance for a range of situations, and a good understanding of the basis of certification and operational requirements to enable application of the information.
My concerns in this instance are that many pilots apparently do not have sufficient understanding of the risks in landing (both in operation and in changing procedures), and possible they may not have appropriate information – factored landing data.
The solutions to these problems are held jointly with pilots, operators, and regulators, particularly the latter for the lack of simple explanatory material.

The UK has made an effort http://www.nats-uk.ead-it.com/aip/current/aic/EG_Circ_2006_P_014_en.pdf and http://www.nats-uk.ead-it.com/aip/current/aic/EG_Circ_2007_P_086_en.pdf, but unfortunately these are not promoted by EASA.
ICAO has recognised the problems of assessment and reporting of runway conditions, hence the initiative linked in #11.

It is down to QRP, isn't it? In this instance I don’t believe that it is.
The ‘Boeing’ QRP advisory landing data can be used to calculate a factored distance, but as discussed previously it may not be the same as the certificated data, and thus may not meet the ‘required’ level of safety. In addition, the need for crews to calculate data as opposed to asses conditions and choose the relevant prepared data, adds workload and invites opportunity for error.
I conclude that in this instance you do not have the appropriate information for the normal operation in your cockpit.

[capt. solipsist]

How short is SHORT? We used to routinely operate in a rwy 1420m long (or short), monsoon or no monsoon, and if your technique is correct, would still have plenty of concrete to spare even w/o reversers.

Sometimes, the operators know more about the acft than the manufacturers.

[BOAC]

I conclude that in this instance you do not have the appropriate information for the normal operation in your cockpit. - and you may well be right, but what do you suggest should be in the cockpit and what was in yours when you operated?

[safetypee]

“… what was in yours when you operated?”
As above (#19). A tabulation of factored wet landing performance for each runway and diversion airport in the route structure (it could be regionalised), and generic tables for emergency off-route operation.
Yes this was a big book, but nowadays it’s probably in the computer.
A dominating problem is not what is in the cockpit, but what the crew may not know about the runway condition, and thus which table is relevant.

IMHO ‘short’ is not the absolute runway length, but the relationship between distance available and distance required. For any combination of weight, weather, runway condition, etc, where actual and required approach equality, then the runway is ‘short’.

One of the problems is that most tabulated data relates to landing weight (mass – as required by regulation). Unfortunately this does not provide the pilot with a good indication of the distances involved (situation awareness).
Thus even when working with factored distances, if the landing weight is within 90% of the maximum allowed then the runway can be considered ‘short’.
This awareness need not necessarily trigger a different mode of operation, but identify increasing risk, consideration of a higher braking level, tailwind restrictions, and specific attention to the runway condition.

In very wet or potentially contaminated conditions (SOP was to avoid), the landing briefing should consider the next worst case runway condition, i.e. for a very wet landing (heavy rain), the crew should be aware of the margins available for a flooded runway (error management). The 'flooded' factored distance required may exceed the runway length; the crew could then subtract a factor to check what margin there might be, albeit with a higher risk. This a defence for the ‘what ifs’ … the spoilers or reverser fail (thread topic), the runway was actually flooded, or the tailwind misreported – never land in a tailwind on a very wet / flooded runway.

Back to the thread.
Any change in procedure would have to consider airborne time/distance (float) and the residual speed. I suspect that the additional distances involved with these would exceed any additional benefit from ‘quick reverse. It’s always better to decelerate on the ground.

AC 91-71 Runway Overrun Prevention. (www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/0052F2A2A00D91B28625738E0071E44C?OpenDocument&Highlight=ac%2091-79)

Managing Threats and Errors during Approach and Landing (http://www.skybrary.aero/bookshelf/content/bookDetails.php?bookId=874)

[ant1]

As TE (initial poster) said, according to FCTM, TL should ideally reach the closed position at touchdown.

I personally don't like the idea of touching down with a significant amount of thrust.

From FAA advisory circular: However, the pilot should not command significant reverse thrust until the nosewheel is on the ground.

The FCTM states to deploy TR after main wheel touchdown (Automatic Breaking) and for Overweight landings specifically says not to wait for nose wheel touchdown.

As anther poster said there are other tools. Flaps 40º being one. In my outfit they are allergic to that possibly because of auto retraction and roll tendencies on earlier models which we don't have but that is another story and a thread digression

And finally the old adage: if Boeing do not specifically tell you to do so, don't dot it.

When I integrate all the above I come to the conclusion that 1) using the right Auto Bake and then max effort if required 2) arming and monitoring SB 3) having the TL closed at touchdown 4) commanding reverse thrust ASAP should do the trick.

[Tee Emm]

Use of reverse as soon as possible after main wheel touchdown in order to take into account the greater efficiency of reverse at high speed. We did some amateur (by test pilot standards) un-measured two engine rejected take offs in a B737-300 simulator using wet (slippery) and dry runways and at 61 tonnes. The purpose was to see how much reverse was worth on a slippery runway compared to a dry runway.
We did four rejects. The runway was 11,000 feet sea level ISA so no problem with balanced field length and lots of runway to spare Reject speed for Flap 1 was 150 knots.
First reject using RTO autobrake manual speed brake and no reverse. Pulled up with a mile to spare.
Second reject, manual speed brake, no brakes and reverse only and we went off the end at 45 knots.
Third reject slippery runway, RTO and manual speed brake and no reverse and off the end at 45 knots.
Fourth reject slippery runway, manual speed brake, RTO and full reverse and we stopped with about 1500 feet to go.

The conclusion we reached was that on a slippery runway the use of full reverse made all the difference between pulling up 1500 ft from the end and going off the end at 45 knots with no reverse. Whereas on a dry runway reverse is only worth around 170 ft according to the FCTM tables. The quicker you got into full reverse the shorter the pull up distance.

Another difference was the actual N1 attained on rundown on both engines at the time we got the reverse levers through the interlocks and that was about 55 - 60 percent N1 which meant quicker spool up - which was the point of the original post.
During my time on the 737 operating into relatively short runways, I used standard Boeing technique for thrust lever operation and applied reverse also as standard. The originally post was thrown into the ring as a discussion area - never as a substitute for Boeing recommended procedures

[BOAC]

A tabulation of factored wet landing performance for each runway and diversion airport in the route structure (it could be regionalised), and generic tables for emergency off-route operation.
Yes this was a big book, but nowadays it’s probably in the computer. - small route structure or large cockpit? Amazing.