Gliding Descent to Save Fuel

[ReverseFlight]

New 'gliding' landing method for planes to slash fuel consumption (http://www.theage.com.au/travel/travel-news/new-gliding-landing-method-for-planes-to-slash-fuel-consumption-20090811-eg53.html)

Aviation group Scandinavian Airlines System said on Monday it had designed a new landing method for aircraft, which could slash fuel consumption and emissions of carbon dioxide. The new technique, which involves planes gliding into land following an optimum route mapped out by satellite, could save around 100kg of fuel in a twin-engined jet, the group said ... The group believes the method would be best suited to quieter airports which are surrounded by hills or mountains.

I am curious as to how this really works in practice, and what limitations there are in using this technique. Any advice appreciated.

[muduckace]

Hell, the most economic descent would be so but unless they have a commercial glider in mind it will never happen.

Having said that, for ATC to calculate descent into a straight in approach "engines idle" (complex calculations given aircraft type and weight) would be ideal but unprecedented. As aircraft enter the approach phase thrust is required to offset the increased drag normally to safely land at design speeds.

[Capn Bloggs]

All that is is a Continuous Descent Approach. No big deal at all. Every modern jet can do them at near idle power. In the 717, a small amount of "fat" (too much, in my opinion) is built in so that if unexpected tailwinds are encountered, the aircraft can still stay on/get back to the ideal descent profile and to provide a safe approach. Spinning up from Flight Idle just before touchdown is not desirable.:eek:

The big problem is ATC and other traffic getting in the way, requiring either lower-than-desired profiles, non-optimum speed or hold-ups.

If one operator does successful CDAs, then another is being unfairly penalised because they are being kept out of the way.

I believe all this bluster about CDAs and RNP is just a smokescreen for an ATC system which cannot cope with the traffic levels.

[Capetonian]

Referred to here : http://www.pprune.org/jet-blast/384690-jets-land-neutral.html

as 'landing in neutral'!

[Henry VIII]

Since a cuple of years Airbus and Boeing in cooperation with some airlines are testing in the USA at SFO a sistem granting a CDA in congestionated areas also.
ATC send an "approach profile proposal" to the aircraft by CPDLC, the crew evaluate the proposal and, if they accept, download and activate it.

Text reference here. (http://www.aviationsystemsdivision.arc.nasa.gov/publications/enroute/eda/coppenbarger_09_07.pdf)

[RLV1AD]

ReverseFlight, you can read some more here:

Arlanda and Brisbane Airports Pursue RNP and 4D Trajectories | AVIATION WEEK (http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=awst&id=news/aw042808p1.xml)

SAS Performs First Managed 4D Trajectory Flight in Revenue Service (http://www.flttechonline.com/Current/SAS%20Performs%20First%20Managed%204D%20Trajectory%20Flight% 20in%20Revenue%20Service.htm)

[Todders]

Can i just point out the reason we go down final app with power on is not to offset the effect of drag in fact the opposite is true the drag is to offset the thrust.

Reason for this is jet engines require time to come up to power, if you do not have them stablised the response time in the event of a go around(Idle thrust to go around power in the event of a "glide app") would be unacceptable at least with decision heights that are normal/required for all wx airline transport ops.

Todders

[Henry VIII]

To perform a CDA does not mean to land at idle thrust.
Each company has an altitude gate at which acft MUST be stabilized accordingly to different wx environment. And one of the stabilizing parameter to be respected is thrust out of idle.

[con-pilot]

When I was flying the 727 we had a contest between the pilots on this subject. The contest, when the weather was VMC and ATC traffic load low, was two fold, first that when cleared from for descent at pilots discretion from the mid altitude, around FL 240, you closed the throttles, and timed the descent to hit 10,000ft. and 250 kts IAS at the same time. Then, again with the weather and ATC cooperating, you would continue the descent to arrive at the OM, or if no ILS a five mile final, at glide slope intercept altitude, start extending the LEDs/flaps/gear and then at 500 feet in landing configuration you would touch the throttles bring up the power for landing.

After a while it became quite easy. Oh, you were allowed to use the boards, but if you did, you only won one beer.

I landed the 727 only one time never touching the throttles from 10,000ft and it was not intentional. I think I was set up by the guy in the right seat and the FE. We were landing at NAS Miramar (of 'Top Gun' fame), I had coasted in from the west for a right hand downwind. I was at flaps 15 still descending at the midfield point when I was distracted looking for a pair of F-14s that were called out to us as traffic. After I finally located the F-14s I realized that I had passed the end of the runway, how far I didn't know. So I ask my good buddy in the right seat how it looked for turning base, this was the following conversation, sort of;

Guy in right seat=RS

FE=FE (duh)

Me=Me (of course)

RS, "It looks fine, go ahead and turn in."

Me as I look at him, "You sure?"

RS, "Oh yeah, it's fine."

FE peering out the right side cockpit window, "Oh yeah, you can make it."

Me, I start the turn and call for 'gear down'. (Still at flight idle, where the engines had been since we left 10,000ft.)

Me when I finally see the runway, "Jesus Christ, we're too close, flaps 20 then on speed flaps 30, before landing checklist at flaps 30."

RS, "Huh, yeah, I just might have called that just a mite too soon. Flaps moving."

Me, "No sh!t.'

At this point I place my hand on the throttles, more out of habit in this case instead of need.

Me, "Okay, we will touch down in the touchdown zone, but at what airspeed I've not a clue, but we have 12,000ft of runway so stopping will not be a problem, you two a$$holes."

We landed without me ever moving the throttles, stopped with moderate braking with 5,000ft of runway left. That night in Phoenix where we RON my two 'buddies' bought all my beers that night.

So the point of this post is, this idea can and will work with weather and ATC cooperating. However, I don't have clue if an fly-by-wire Airbus can do total flight idle descent from altitude to landing.

[Wizofoz]

And, of course, "Slashing" fuel aqnd CO2 is a total load of carp. If we could engineer CDAs on every approach we would make a very usful saving of a few hundred kilos per flight- maybe .5% of total fuel burn, and well worth doing.

But hardly "Slashing":rolleyes:

[Microburst2002]

Long time ago there was a 727 accident caused by a idle landing.

Idle landing is dangerous for all swept wing airplanes.

[hawk37]

Microburst,

What is it about swept wing airplanes that make an idle landing dangerous, compared to non swept wing airplanes?

[411A]

Reason for this is jet engines require time to come up to power, if you do not have them stablised the response time in the event of a go around(Idle thrust to go around power in the event of a "glide app") would be unacceptable at least with decision heights that are normal/required for all wx airline transport ops.


Used to be this way on older designs, however....if one has three shaft Rollers installed, these engines spool-up mighty fast.
Example.
Lockheed during certification flying for the L1011, a flight idle approach to 200 feet AGL was tried several times, yet the engines spooled-up just fine for a go-around.
And yes, I know the guys that 'done it.'
Cannot be used in commercial service, of course, but demonstrates the adaptability of the Roller's three shaft design.

RollsRoyce...builds good turbine engines:ok:
A superb design...quiet, too.

[Microburst2002]

hawk 37

It has to do with the slope of the CL / AoA curve. In a 727 it is much shallower than in an ATR. Swept wings are good for flying high and fast without having too much shock wave drag, but they are bad for flying low and slow because they "waste" some lifting ability.
If you compare swept and non-swept wings lift versus angle of attack curves you will see that for a given change in angle of attack, a 727 would have less increment in lift than an ATR.
You are in a 727 at 50 ft above the threshold at idle with too high a rate of descent and, because of the idle condition, with a steeper than usual angle of descend. You will have difficulties in arresting that rate and that angle with elevators only, as a change in pitch will not have much effect in lift, and therefore in the flight path. Besides, drag increases a lot during a pitch up at landing speed. So instead of a normal flare and landing the result can be a hard landing or even an accident.

On top of that, accelerating the engine to get the thrust you need to solve the problem can take too long, so you may be unable to avoid the plane from falling on the runway or even you could instinctively and frenetically pull the nose till the stall, only worsening things.

In an ATR, pulling up the nose can quickly change the flight path from steep to shallow. You can also increase thrust faster.

[Capt. Inop]

I am curious as to how this really works in practice,

Well coming from south for rwy 01 works great, can keeep em at idle until ya gottabe stabilized.

19 is worse at least from south.

Guess SAS suddenly figured out that their NG's have GPS input and got procedures to deal with it.


(journos must have a dull time)

Have more landings at ENTC than i like to think of.

[Capt. Inop]

Referred to here : Jets to land in Neutral...

as 'landing in neutral'!

One have to wonder, why is that thread moved to or created in
jet blast? :}:cool:

[hawk37]

Microburst,
If we can, lets keep the jet engine vs turbprop discussion aside. Swept wings was the specific topic.
I understand what you have said about the CL/aoa curve, just trying to relate that to an aircraft at Vref, for old times sake lets say 1.3 x Vso.
Now, it doesnt matter if we're a swept wing aircraft or straight wing, in the situation I asked, the aircraft is at Vref presumably, at dh descending to 50 feet or so for the landing.
You've got a better grasp of this than I have, so bear with me for this...
Regardless of whether it's straight wing or swept wing, there's still this 1.3 margin in speed over the power off stall speed. So...either aircraft at that speed can generate 1.69 times it's lift (L proportional to V squared). Now I know MFS is going to chime in here and say it's really 1.23 squared, but it's the concept I'm trying to demonstrate.
Basically, this means at the extreme, swept or non swept wing aircraft both have the same 1.69 G capability. So, why is hard to land a swept wing aircraft vs non swept at idle thrust?
If it's really then because of the added drag, as you say, of the swept wing aircraft at higher aoa? If so, push up the thrust levers!!
Beer number 4. Hope I'm making sense

[Old Fella]

Nothing wrong with landing with engines back at idle if speed is correct. Doesn't matter whether it is a 'straight' wing or a 'swept' wing, they both need the approach speed to be correct. As others have said, the danger in being back at idle can be the time required to accelerate the engine if needed.

[Microburst2002]

Hawk 37

If I had not read about that accident and its causes I would have doubts now, because what you say makes sense.
Let me think while writing.
Let us assume the airplanes (swept and straight wing) are at 50 ft above the threshold, idle power, and Vref. So they both are at 1,3 Vs, they have the same stall speed margin.
However, the angle of attack still plays a role in lift procuction. In a swept wing airplane, for each extra degree of angle of attack you get less extra lift than in a straight wing airplane. Both airplanes are equally away from stall but they have not the same ability to manoeuvre, to change trayectory. The swept wing CL-AoA curve is flatter. Margin to stall is one thing, ability to change flight path is another. By the way, the swept wing has a higher stall angle of attack (you can see it comparing both curves) I am not sure is this has an influence, too but I don' think so.
If you can take a look at the curves, the swept one has a lower CL max but a higher CL max AoA. Less CL means less lift for a given speed. So the conclusion is that the swept wing airplane has less CL available than the straight wing plane.
As a matter of fact, if you are making an idle landing in a swept wing airplane you better have a fiew extra knots (LDA permitting). Usually, the reason for an idle landing is that you haven been able to decelerate to VREF. But if you just reach that speed when about to flare still in idle...

[Capn Bloggs]

Old Fella,
Nothing wrong with landing with engines back at idle if speed is correct. Doesn't matter whether it is a 'straight' wing or a 'swept' wing, they both need the approach speed to be correct.
If you mean doing the approach and landing at idle, you're mistaken. Every jet operator I know of has a strict stabilised approach SOP which requires the aircraft to be between Vapp-5 to Vapp+15 (or less) and stay that way from 500ft AGL (some 1000ft AGL) to approx 50ft on the 3° slope. There is no way in a modern jet one could be stable and remain so at Idle.

Anybody who seriously thinks idle descents to touchdown in a jet is feasible doesn't fly jets. "Gliding in to land" means idle power until 500ft at the very latest.

[deagles]

NASA do it all the time but the technology is expensive.

[Bergerie1]

Constant Descent Approaches (CDAs)

CDAs are nothing new; we have been doing them for years. They are no more than descents from TOD at idle power to the start of the approach. And while we are at it, let’s kill stone dead the idea of glide approaches to touch down in commercial operations! The CDAs that SAS and others have been doing all require the aircraft to be stable (Vref+ the correct increment, gear down, landing flap, and power set) at or before 1000ft agl. The problem is how do you do this in a complex and busy ATC environment without reducing the overall capacity of the system?

Recently, several new technologies have been tested in simulations and line operations. The first is ADS-B which uses the Mode S transponder to broadcast information on the aircraft’s position and intent. ADS-B ‘Out’ can provide ATC with better information than current radar. If ADS-B ‘In’ is fitted as well, the aircraft can receive this same information and display it in the cockpit. Page 37 to 41 in the Flight Safety Document on this website will explain more:-
http://www.flightsafety.org/asw/jul09/asw_jul09.pdf (http://www.flightsafety.org/asw/jul09/asw_jul09.pdf)

The FAA in their work on NextGen and the Europeans in the SESAR Joint Undertaking are working towards the same ends. The current problem is that ATC do not know what the aircraft is doing and the pilot does not know where he/she is fitting into the traffic pattern. Modern FMS has much better information of the aircraft’s position and projected trajectory than anything ATC can conjure up. So, if this can be data-linked to ATC it would become possible for them to fit aircraft much more elegantly into the overall system and thereby improve safety, reduce workload and improve economy. Equally, if the pilots could see where they are in relation to other traffic in the descent and approach pattern, they could have certain tasks delegated to them by ATC, e.g. to follow in the stream at a constant distance or time interval from preceding aircraft. Eurocontrol and other research bodies have done extensive simulations using active line pilots and active controllers and have proved it can work. Also, UPS at Louisville in Kentucky have been using the system in line operations. See the presentation 6 at the ASAS Global Network Forum in this website:-
http://groups.google.com/group/asas-global-network?hl=en (http://groups.google.com/group/asas-global-network?hl=en)

I have also flown similar systems in simulations at NLR’s facilities in Amsterdam and can vouch for their potential. So, let’s remove the concern about glide approaches and see how the whole ATM system could be improved to increase safety, capacity, economy, and reduce environmental emissions.

[hawk37]

Microburst,

you say "Both airplanes are equally away from stall but they have not the same ability to manoeuvre, to change trayectory"

Not sure I agree with that. Both aircraft have a 1.69 G capability to maneouvre. If they are at the same tas, that's the same turn radius

I don't see what higher coefficents of lift, flatter aoa curves, more aoa change required to add lift etc have to do with this argument. Your points about that may be true, but a 1.3 x V stall factor should equate to same maneouvrability, flare capability etc as long as the aircraft are at the same tas.

I'm not suggesting the final approach/landing be done at idle power, and I appreciate the spool up times required, and perhaps the added drag of swept wing aircraft at high aoa. My inquiry was about the ability to arrest rates of descent, as I believe you put it in one of your earlier posts, of swept wing aircraft versus straight wing. I don't see why they are different.

[Microburst2002]

Hello Hawk

The truth is I don't know where that 1.96g comes from.
As far as I know, gs come from Lift. If one airplane has less ability to increase its lift than other airplane, all other factors remaining constant (weight, speed...) it follows that this airplane has less manoeuverability.
Please I would like to see how you get the 1.96 g in detail.
Regarding the increased drag, that is another reason not to advise idle landings in jets, and most swept wing airplanes are jets. Jets Vref is closer to the reverse command region than props' Vref. This means that we have less speed stability and we can easily fall into that region if we raise the pitch too much without adding thrust positively. Haven't you ever been a few knots below Vref and have added a little bit of thrust to recover, but it didn´t suffice, and then added some more, with little result, and then more and more thrust until you realized that the thrust was well above the normal final approach thrust? That is the speed instability. If only you had added more thrust the first time you would have gone out of the limits of the unstable speed region, but little by little you remained there a long time.
Imagine an idle landing in that situation, a few knots below Vref (it can happen if actual weight is above computed weight, among other reasons).
As I said (well, as the 727 accident report said) it is no good idea to do an idle landing in a swept wing airplane.
Summing up, I think the reasons for discouraging idle landings in jets (with swept wings) are:
- higher sink rate
- steeper path requiring more manouverability for the flare
- reduced manoeuverability (here we disagree)
- speed instability (or little speed stability)
- possibly excessive time to spool up jet engines

Nice to review and discuss about this subject! I have to take a look at a few books I don't have right now to check what happens to AoA, g and speed when speed is 1,3 Vs.

[Microburst2002]

Hi again, Hawk

I have been working with a pen, a paper and my old fx-82 (no books, unfortunately) and damnit! I think you are right ;)
If at stall speed (1g) we have a given lift coefficient then at 1,3 that speed (1g) we have 0,59 times that lift coefficient. Irrespective of the type of wing. Which means that if at approach speed (1g) we have a given CL , increasing CL to CLmax we will achieve 1,69 times that coefficient: 1,69 g capability, as you said!
The swept wing only means we need more angle of attack to achieve the CLmax. This fact could contribute to the unstable speed regime risk that I mentioned earlier, as a large increase in AoA means a large increase in drag (but how swept wing affects CD as compared with a straigh wing one I don't know. Argh!).
Maybe the risk is tal strike in swept wing airplanes when idle landing, rather than inability to flare
It also means that, since CLmax is lower, Vref in a swept wing airplane nees to be higher than in a straight wing one, but that only contributes to require more distance to land.
Still, we have some reasons why we should not attempt idle landing in jet airplanes. And I swear on god that the report I read showed the damn swept wing CL-AoA curves!

Thanks. I have enjoyed learning!

[Keith.Williams.]

If we are talking about gliding all the way to the ground then we will end up with a glide slope that is considerably greater than the usual 3 degrees.

Going back to the basics,

Glide range = Lift to Drag ratio x height

Rearranging this gives Lift to Drag ratio equals the cotangent of the glide slope.

For a glide slope of 3 degrees we need a Lift to Drag ratio of about 19 to 1.

Not many commercial transport aircraft will achieve this in the landing configuration.

[TheKabaka]

LHR monitor CDA approach from 6000'. Based operators achieve a CDA 95%+ of the time (not using gps/rnav).

[Bergerie1]

The Kabaka,

It is not much use only to be able to do CDAs from 6000ft. What is needed is for all aircraft to be able to do them from TOD to around 1500ft to 1000ft without intermediate level outs and without overall loss of airport capacity. The systems I gave references for in my last post may be able to achieve that one day. In fact London Heathrow has nothing to boast about; according to the Eurocontrol Performance Review Report for 2008 (www.eurocontrol.int/prc (http://www.eurocontrol.int/prc)) on page 59 in Fig 72 it is shown to have the worst performance, in terms of additional time spent on the approach, of any major airport in Europe!

[411A]

London Heathrow has nothing to boast about; according to the Eurocontrol Performance Review Report for 2008 (www.eurocontrol.int/prc) on page 59 in Fig 72 it is shown to have the worst performance, in terms of additional time spent on the approach, of any major airport in Europe!


Opps...another LHR hurrah down the drain.:}

[org]

The reason the 727 had high sinkrate accidents early in its existence had nothing to do with the sweep of it's wings and everything to do with the huge amount of drag generated by the LEDs and flaps in the landing configuration. Basically, as soon as the nose comes up, the speed goes away....fast, and without really changing the descent path of the airplane. If the engines aren't spooled there is no margin for error and even then a late application of power can take what seems to be eons to change the descent path. Being below a couple hundred feet, unspooled, at Vref is a dangerous place in a 727. A fantastic airplane, but it had to be flown like a 727, not just a "swept wing airplane".

The DC8, on the other hand also had a swept wing, was much larger and heavier, and had no unusual high sinkrate tendencies. It also had a different wing section and no LEDs, as well as much smaller (relatively) flaps. It had a much larger margin of error.

[freqslf]

RYR STN->BTS, two years ago (737-800NG, august, very nice weather, no wind, excellent visibility, ac nearly empty) - the pilot made a descent from (just guessing) 2-3 km, and the engines didn't spool up till we were taxiing. Extremely smooth approach and landing, no reverse thrust and very little braking. The best landing I ever experienced!


(I fully understand it is not possible for a slf to be sure about engine RPM, and I am not saying they were idling, just that I couldn't hear them)

[Capn Bloggs]

the pilot made a descent from (just guessing) 2-3 km, and the engines didn't spool up till we were taxiing. Extremely smooth approach and landing, no reverse thrust and very little braking. The best landing I ever experienced!
Based on my experience, that approach would have been unstable by a long shot. It is not possible, in a modern jet, to fly from 500ft AGL to touchdown with the engines at idle (which it sounds like they were if they had to be "spooled up" for taxi) within the stabilised speed limits. I therefore conclude, based on the info given, that that crew hit 500ft going way too fast and bled off the speed down final.

Don't judge an approach and landing by the touchdown...

[By George]

Thanks Bergeriel, you have it in a nut shell. Kieth williams as an ATPL Instructor you surely must know gliding to touch-down will never be part of the equation due to 'stable approach' requirements. As Bergeriel states CDA is and has been a standard method of descent since jets were invented. What stops a 'perfect' idle descent being ATC requirements. The only thing that is changing is trying to get ATC more on board. With the traffic density the way it is I have my doubts, but it's worth a try. As for the 727, if any aeroplane needs power to land, thats the one. You never closed the thrust levers until you were finished with the wings.

[freqslf]

Dear Captain Bloggs,

I appreciate fully that a "smooth landing" from a slf perspective isn't necessarily a "technically clean" landing. I just wanted to point out that it happens (probably a very rare occasion) - and that in fact you can glide to landing on a 737 (in ideal conditions). We may also consider the fact that BTS isn't a big or busy airport. Also, arriving from STN, the ac didn't come directly to landing, but made a wide turn before aligning and coming to land (already on idle). I am guessing the pilot had a clearance for a visual approach, and did this intentionally. He may have flown a bit faster to the landing, but as the ac was nearly empty, it didn't lead to some heavy braking.

I am eager to see if i'll ever have the chance to experience this kind of "quiet landing" again. It felt like being in an oversized glider! :ok:


(humbly going back to "read-only mode" ;-)

[PappyJ]

Personally, I'll sacrifice a little fuel to be fully configured, with all checklists completed, etc, before 1000 feet AGL. Just feel safer that way.

But, for interest, have a read of the Hong Kong Noise Abatement procedures for late night - early morning arrivals; "...all operators...CDA approach....thrust idle....until touch down..."

[Microburst2002]

And, if safety not compromised, with engines cut off!
haha

[Black Knat]

If you watch closely the landings of the (swept wing) Space Shuttle as it lands I suppose you'll see the engines spool up @ around 500'........? Guess that's why it doesn't crash into the ground each time it lands???!

[hawk37]

FreqSLF,

I would surmise that the additional airstream noise and vibration from having landing gear hanging and flaps at full deflection for landing was enough to mask the sound of the engines at approach power. For the jets I've flown, the engine's rotor speed has been in the 60 - 70 % N1 for the approaches, and this is usually much quieter than the up to 100 % one might hear on takeoff or even climb.

[Capn Bloggs]

The space shuttle does not have any operative engines during re-entry and landing. That's why it's descent angle is so steep; it IS gliding - like a brick. The 500ft thingee is the commencement of the flare for landing.

[By George]

Black Nat, have you any idea on the profile of the Shuttle? 1500 feet/second initially and a final profile of 10,000 feet at five miles. Sure we can all fly like that and land like that, with training, but its dangerous and we don't do anything dangerous with the general public down the back. Your 'Space Shuttle' theory also has a few holes in it with a Go-Around.

[Black Knat]

Does the space shuttle not use reverse thrust to stop then?

[G-V]

As far as I know, space shuttle is not using any thrust/engine after reentry. It glides only.

[777den]

On landing the Space Shuttle has 3 braking systems: split drag brake on the tail , wheel brakes, drag chute. It has NO fuel left for the engines and NO way to abort the landing and go-around. Once they fire the OMS engines to de-orbit they are committed to a glider landing.

[slamer.]

Therefore all carrier landings (yes.. they use swept wing jets too) should be a glide approach to TD....:rolleyes:

[Microburst2002]

If we always had unlimited runway lenght, there would be no problem in CDAs.
Just make the approach at a few knots above Vref, make an early flare, float for a while, watch pitch to prevent a tailstrike and touch down nicely.
I am sure we all have done this a few times after a visual approach (when runway available was not an issue).
I guess that is what the space shuttle does, basically

[ReverseFlight]

A postscript to this thread I started - the press finally realises that Qantas is in the game:
'Perfect flight path': Qantas expands GPS use (http://www.theage.com.au/travel/travel-news/perfect-flight-path-qantas-expands-gps-use-20091203-k7yy.html)


Qantas is expanding its use of the latest generation GPS-based navigation systems to some Melbourne flights that promises "a perfect flight path, gate to gate ", which is expected to save oceans of fuel, tonnes of greenhouse gases, flying hours, time and money. From today, Qantas 737-800s flying in and out of Melbourne will be using a cutting-edge flight system called "RNP" — Required Navigational Performance.

[kenparry]

Is it not wonderful what some PR bullsh*t can do. Nothing new about RNP, it's been around not just for years, but for decades.

Oh well.