Ask The Pilot

[jb747]

Does that mean you got the chance to fly RNP approaches too? They are great to fly and the accuracy is unbelievable.

Interestingly enough the 777 only got approved about a month ago to fly RNPs and we had been doing them for years.

We flew the RNAV STARs, and then they had to join some form of approach for finals, so that's not quite the same thing. We could do GLS autolands though. From what I understand, an RNP approach would integrate both STAR and approach phases into one item, with the aircraft in APP mode for the entire event. We couldn't do that.

Looking at a Jeppesen...we could do the GNSS but not the RNP.

 

[jb747]

Is this similar to what EK tried to do at JFK?

At the end of any approach, the remaining part of the flight is to be continued visually. They were trying to circumvent that, by building a series of 3D waypoints into the FMC. This is specifically banned by Airbus....if the approach is not coded (by Jeppesen) into the FMC, then it must be flown using raw data. If what was done, had only been one crew, then it would be bad airmanship. But, it was apparently an EK norm, so that's really asking to feature in the news.

 

[Quickstatus]

How does the pilot monitor the performance/accuracy of such "auto approaches/departures" like RNP?
Presumably the more accurate RNP is the more aircraft can fit into a volume of air without interfering with legal separation?

 

[747sp]

If Qantas was facing the 737 max problem or the Air New Zealand issue with the 787 and the engines and had the planes grounded and decided to bring back one of its 747s from storage to fill the gap what issues would be faced?
For example I understand bring OJS back technically would not be hard but I imagine that staffing would be an issue as pilots would have moved on to other types and the d check would also be a limiting factor.
What I am really interested in is the factors from the pilots perspective that makes it hard to bring a plane back to service.

 

[Quickstatus]

Is there such a thing as an inherently stable aircraft?

Can instability be positive? Maybe miitary fighter aircraft need to be more unstable as it can allow more unusual movements necessary in say a dogfight?

If 737M8 by design an inherently unstable aircraft under certain circumstances - greater pitch up moment due to position of more powerful engines at certain pitch up angles, then is it good engineering to use control surfaces as an antidote?

 

[AviatorInsight]

How does the pilot monitor the performance/accuracy of such "auto approaches/departures" like RNP?
Presumably the more accurate RNP is the more aircraft can fit into a volume of air without interfering with legal separation?

RNP is more about lateral navigation than it is vertical. To fly an RNP SID (HBA) you need to have an accuracy or 1nm or less coming from the satellites to the GPS.

The approach is where things get good. So we usually fly to an RNP approach of 0.3 (the tolerance is only 0.3nm wide). Vertically it is coded into the FMC all the way down to 50ft above the runway threshold. If conditions dictate it then we are approved to make that tolerance 0.11nm wide. The A/P does a great job of flying it but it means the PM must be making sure the aircraft is within tolerance the entire time, that the check height at the final approach fix (FAF) is on slope and that the FMC changes from Approach mode 1nm tolerance to terminal mode (0.3 or 0.11) by the FAF.

RNP isn't about fitting more aircraft into a volume of air, it means they have the flexibility of curved approaches (CBR, OOL, MCY etc) to help with noise abatement, rather than the straight ILS. If they want more traffic, then BNE actually utilises the ILS for traffic management because there is more spacing along final.

 

[Quickstatus]

So how does the PM check if the RNP approach/departure is within tolerance? 0.3nm is not a lot of distance about 500 metres and not a lot of time to correct in a moving aircraft

 

[AviatorInsight]

Is there such a thing as an inherently stable aircraft?

Can instability be positive? Maybe miitary fighter aircraft need to be more unstable as it can allow more unusual movements necessary in say a dogfight?

If 737M8 by design an inherently unstable aircraft under certain circumstances - greater pitch up moment due to position of more powerful engines at certain pitch up angles, then is it good engineering to use control surfaces as an antidote?

Yes. The more stable an aircraft is the less maneuverable. Take the C172. They are very stable, suitable for student pilots. But you can't make a 172 do very rapid changes. Aerobatic aircraft and fighter jets can respond to pilot inputs very rapidly, but controlling one requires much more skill.

In any aircraft with underslung engines on the wings, you will get a greater pitch up moment anytime thrust is applied. But the controls are able to counteract that pitching force and then using trim to relieve the pressure.

 

[AviatorInsight]

So how does the PM check if the RNP approach/departure is within tolerance? 0.3nm is not a lot of distance about 500 metres and not a lot of time to correct in a moving aircraft

Down the bottom of the ND is says RNP (required navigation performance) and ANP (actual navigation performance). If ANP<RNP all is good. The A/P does a great job of it but anytime ANP>RNP a missed approach must be performed in IMC. If we're visual we'll just change to a visual approach.

The picture below is in the cruise (along with 6 other aircraft) the RNP for the cruise phase of flight is 2nm but the FMC is achieving 0.07nm. Pretty accurate!

 

[Quickstatus]

How does the aircraft know the error is 120metres?
Can the same accuracy be obtained if the route is flown manually?

 

[jb747]

How does the pilot monitor the performance/accuracy of such "auto approaches/departures" like RNP?

The achieved value is displayed on the FMC. If needed for an approach, the system will automatically load the required value as a limit (and we can do the same). Failure to achieve the required value will cause both a warning and the system reversion.

 

[jb747]

If Qantas was facing the 737 max problem or the Air New Zealand issue with the 787 and the engines and had the planes grounded and decided to bring back one of its 747s from storage to fill the gap what issues would be faced?
For example I understand bring OJS back technically would not be hard but I imagine that staffing would be an issue as pilots would have moved on to other types and the d check would also be a limiting factor.
What I am really interested in is the factors from the pilots perspective that makes it hard to bring a plane back to service.

Ignoring any engineering issues, it would all depend upon how long it had been since your pilots had flown the aircraft, and to a degree, what they'd been flying in the interim. Less than 6 months, you could get them back up in a week or so. A year, perhaps a month.

Mind you, I last flew 6 weeks ago, and I think I'm forgetting the details already.

 

[AviatorInsight]

How does the aircraft know the error is 120metres?
Can the same accuracy be obtained if the route is flown manually?

The aircraft gets its position from the GPS (satellites). If the integrity of those satellites decreases then the ANP increases.

I've tried to fly it manually (on a visual kind of day) and it is very hard to do when the wind kicks in at lower level or there's shear. I have flown it to within 0.3nm but the A/P usually sits at <0.1nm. But if there's any doubt, go around and try another approach where the tolerance is greater.

 

[jb747]

Is there such a thing as an inherently stable aircraft?

The vast majority of aircraft are stable in all axes. An unstable aircraft cannot be flown without a computer interface. Even a neutrally stable aircraft will present a lot of difficulty.

Can instability be positive? Maybe miitary fighter aircraft need to be more unstable as it can allow more unusual movements necessary in say a dogfight?

Positive stability simply means its stable. Disturb it, and it will tend to return to the start position. Unstable means that it will diverge, and the rate of divergence will increase. Fighters aren't necessarily unstable, but may be neutrally stable, or slightly unstable, in pitch. In roll and yaw, I can see no advantage real advantage, though fighters always have more roll response than any civil aircraft...by a factor of about 50!

If 737M8 by design an inherently unstable aircraft under certain circumstances - greater pitch up moment due to position of more powerful engines at certain pitch up angles, then is it good engineering to use control surfaces as an antidote?

The MAX is not unstable. The issue, as best I can tell, is that if you get near the stall with full power applied, the elevator does not have enough authority to get the nose down, or even to stop the pitch up. I'd have thought a reduction in the power applied might have been a much easier solution.

 

[jb747]

Can the same accuracy be obtained if the route is flown manually?

If the information is being presented via the flight director, then you should be able to fly it reasonably, but it's something that you'd need to practice regularly. In theory that would make it little different to an ILS...but, from what I've been told, you really need to have a HUD to have any chance of consistently flying with the accuracy required.

 

[Himeno]

If Qantas was facing the 737 max problem or the Air New Zealand issue with the 787 and the engines and had the planes grounded and decided to bring back one of its 747s from storage to fill the gap what issues would be faced?

Unless they knew it was going to be a long year+ issue, they wouldn't look to bring any aircraft back. They'd wet lease some (as they did when they grounded the A380s after QF32 and leased BA777s to run the SIN-LHR legs).

 

[lastrow]

I'd have thought a reduction in the power applied might have been a much easier solution.

Would the auto throttle sense the problem and reduce power to compensate?

 

[AviatorInsight]

Would the auto throttle sense the problem and reduce power to compensate?

If the aircraft is approaching the stall and the auto throttle is still engaged, it's targeting a speed. So it'll actually go full thrust exacerbating the problem.

 

[jb747]

The MAX is not unstable. The issue, as best I can tell, is that if you get near the stall with full power applied, the elevator does not have enough authority to get the nose down, or even to stop the pitch up. I'd have thought a reduction in the power applied might have been a much easier solution.

A little bit more on this. Apparently there is another part to this equation. In some high angle of attack cases, the nacelles of the engines provide sufficient upwards force to exacerbate the loss of elevator authority. This doesn't push into the unstable region, though it would seem that it was not flight tested.

 

[jb747]

Obviously we're all reading whatever we can find about the MAX, and other Boeing aircraft, in the light of events of the last couple of days. I must admit that I find myself conflicted in my feeling for Boeing. The 747 and 767 were awesome aircraft, with incredible records over a long period. The 777 is a wonderful machine, and I really wish QF had gone that way back around 2000.

The 737 though, is really a relic. Every time I've ridden in the coughpit of one, I've been left wondering what era I'm in. Boeing's failure to properly update the coughpit has left mix of items that would have been familiar to a pilot in 1963...mixed with fancy new tech. The great god, commonality, was supposedly the reason for this design stasis, but they would have been so much better off if they'd told Southwest to go jump. Now we are seeing software fixes on an aircraft that is pre FBW...the kludges continue.

Airbus, of course, have their own failings. After almost 10 years of flying one, there are things that I'd change if I could...but, not all that much. The flight control behaviour is safe and generally quite logical..even though some is French logic.