[Jeroen]

Originally Posted by abhishek46

Regarding the altimeter, do modern planes make use of GPS also for altitude?

Dont think so, it is simple not reliable enough when it comes to altitude.

Originally Posted by abhishek46

Also, as you mentioned, pilots have access to both Radio altimeter and Barometric altimeter.
Which one is more accurate and is used/trusted more?

They are used for different purposes. Barometric is used always and drives the altimeter reading on the pilots main display. Radio and or Radar is typically used during (auto-landing) so only on final, close to the ground. It is used by the auto-pilot and you can see it on most cockpit displays becoming active with an annunciation of RA somewhere in the display

See also
https://en.wikipedia.org/wiki/Altimeter

Originally Posted by abhishek46

1. If we have to land a narrow body plane like 737 or 319, then at what distance from the runway do we generally have to line up the plane with the ILS?

It really depends on the approach and if you google some approach plates you can find some idea. I'm no commercial pilot, but most commercial liners even the narrow body one, try to be lined up with the runway some 4- 6 miles out. There are certain approaches/airports that require shorter distances.

Originally Posted by abhishek46

2. How do you guys handle crosswinds during landing? Do you use the rudder or the ailerons? Or combination of both?

There are different techniques, depending on the aircraft, amount of crosswind, pilot's preference and sometime company procedure.

I made several postings on crosswind technique in a different thread. Have a look here:

http://www.team-bhp.com/forum/commer...ml#post3447912

Originally Posted by abhishek46

3. After touchdown, in order to stop the plane, pilots have access to reverse thrust, air brakes on the wings and disc brakes. Is there any order in which they are followed?

What you call air brakes are mostly so called spoilers. They (automatically) deploy as soon as the main gear hits the runway. They don't so much as brake as decrease the lift (They literally spoil the airflow over the wings) in order for the aircraft to settle well on its gear.

Most commercial jets have elaborate auto brake systems. They ensure a certain deceleration rate. As soon as the main gear touches the runway they will increase brake pressure and then reduce speed at a fixed deceleration rate.

The thrust reverses are usually deployed not before the nose wheel touches. On planes that have auto-braking enabled they don't reduce the braking system as ensure less braking is required.

Rule of thumb, thrust reverser get stowed as the plane has slowed down to 80 knots. Below 80 knots they are not that efficient due to low speed and thus low air volume intake and there is a danger of what debris ingress which can seriously damage the engine.

[searchingheaven]

Originally Posted by abhishek46

Regarding the altimeter, do modern planes make use of GPS also for altitude?

No, GPS is inaccurate, and hence is not used for altitude measurement at all.

Originally Posted by abhishek46

Also, as you mentioned, pilots have access to both Radio altimeter and Barometric altimeter.
Which one is more accurate and is used/trusted more?

Some more off topic questions:
1. If we have to land a narrow body plane like 737 or 319, then at what distance from the runway do we generally have to line up the plane with the ILS?

I assume that you want the technical details. So here we go. Pay attention. On the primary flight display, the barometric altitude is displayed. Radio altimeters are used for measurement of altitude for autoland systems and GPWS(ground proximity warning systems.). The decision height is set using radio altimeters. I will demonstrate this using the following photos. These are photos of a B777-300ER set up for an ILS IIIB approach to runway 12L, OMDB, Dubai.

Figure 1
The aircraft is at a altitude of 2680 feet, according to the barometric altimeter, which is set to the local pressure of 29.92 inches. Notice the 'RADIO' label at the bottom right? That label indicates the landing minimums or the Decision height. It is 200 feet for this approach. This is the height at which the we must have adequate visual reference to the landing environment (i.e. approach or runway lighting) to decide whether to continue landing; or go around. For a CAT IIIB approach, DH can go down to 50 ft or less. Yes, it is that accurate and reliable.

Also notice that the localizer is captured at about 11 NM out{DME 10.7- Distance measuring equipment - 10.7 nautical miles}. This means that the aircraft now has lateral guidance towards the runway. But localizer only tells you if you're left or right, not if you're too high or low. Vertical guidance is provided by glide-slopes. If the glide-slope equipment was not working ,we would use Instrument Approach Chart and descent tables.



Figure 2
The radio altimeter comes alive at a height of 2500 feet AGL(above ground level). It's the white text on black background.



Figure 3
At about 8 NM(not shown explicitly in the image), the Glide slope is captured, i.e the aircraft now has vertical guidance toward the runway. The autopilot on the 777-300ER is capable of automatic descent, landing, flare and roll-out. You can see the pink diamonds on the centre line, both laterally and vertically. This means that the aircraft is on the exact descent slope towards the runway and will land on the centre line.

Most of the airports I have flown to, have a G/S capture distance of about 7-10 NM and localizer capture distance of around 15 to 22 NM. This depends on the maintenance of the equipment installed. Also, important is your angle of interception. If you are turning to final at a high deviation angle, then the capture distance decreases. Localizer is generally captured at 13NM within 35° of runway course and 19 NM within 10° of runway course. Glideslope is captured by 10NM within 8° of RWY centreline.




Figure 4
This photo shows the Autopilot of the B777W, going into autoland mode. LAND3 is annunciated when 3 Auto Pilots, 3 Radar Altimeters, 3 ILS receivers, 2 ASA(Associated sensors) are active and working.

PS: Note that there is a difference of 60 ft between the altimeters. This is because runway 12L is 62 ft above MSL.

[Jeroen]

Originally Posted by searchingheaven

No, GPS is inaccurate, and hence is not used for altitude measurement at all.

That label indicates the landing minimums or the Decision height. It is 200 feet for this approach. This is the height at which the we must have adequate visual reference to the landing environment (i.e. approach or runway lighting) to decide whether to continue landing; or go around. For a CAT IIIB approach, DH can go down to 50 ft or less. Yes, it is that accurate and reliable.

Thanks for that excellent explanation and pictures!

Maybe just to add a few more things. You might read about for instance GPS RNAV approaches or similar. In those cases, GPS provides only lateral guidance. Vertical guidance is always done on barometric pressure, or by means of ILS type of systems or such.

It can even be more extreme when it comes to landing, a CATIIIC does not have a decision height. So you land in zero visibility, always full Autoland!

On these 777's (and 747) you can dial in Decision Height (DH) or Decision Altitude, the difference being as described before. One is RA based so actual height above the ground, the other is above MSL.

Jeroen

[anish5225]

Whoa!! So many technologies, so much jargons, so many parameters. And all these as part of your daily job. Suddenly to me the most modern tech laden road car sounds like a humble soul...

[searchingheaven]

Originally Posted by Jeroen

It can even be more extreme when it comes to landing, a CATIIIC does not have a decision height. So you land in zero visibility, always full Autoland!

Thank you for the compliment, Jeroen. Your appreciation means a lot to me.

Just FYI, ILS CAT IIIc has never been used till now anywhere in the world. All the charts I have seen thus far have had CAT IIIC NA in the minimums section. And thus IIIB is the best you can do legally. The only time I have heard of Cat IIIC being used was with the BA TriStars, but even then I think it was a theoretical capability rather than an actual operational one. I believe that they were still required to have an RVR(runway visibility range) of 50m to facilitate taxiing after landing and also to enable emergency vehicles to find them in case of a problem.

Even if the airport has a certified Cat IIIC ILS system, (which would include all the standby power and runway lighting requirements), the actual minima to be applied would depend on the aircraft and operator equipment, training and certification. I've heard rumours that Schipol has CAT IIIc, but I am not too sure about it.

Originally Posted by anish5225

Whoa!! So many technologies, so much jargons, so many parameters. And all these as part of your daily job. Suddenly to me the most modern tech laden road car sounds like a humble soul...

Undeniably, modern aircraft are more complex in their systems than aircraft of a generation or two ago, but all of that complexity exists for a good reason. The keyword is "Safety through redundancy". Aircrafts are seemingly so complex, because they have to be perfectly safe. Unlike a car, you cannot pull off the road to fix a problem. For each and every equipment, there exist couple of redundant backups.

However, I would say that the controls of an aircraft are not complicated, but rather that they are simply foreign to you. I often drive an uncle's BMW and I find it absolutely confusing. I press a button on the side to lower my window and my seat adjusts itself. I press something else and my windows go black etc. This isn't because the car is complex, but it is because I am not familiar with it.

Originally Posted by RVD

Thanks Searchingheaven for the lovely explanation. Did you get the images off the internet ?? Its a really bizzare approach and hence my question.

No, these photos have been taken by me. But in my opinion, Dubai OMDB 12L is a very easy approach. There are lots of visual cues like the Burj Khalifa, the palm islands, the sea itself. And a long final.

[RVD]

Originally Posted by searchingheaven

I assume that you want the technical details.

Thanks Searchingheaven for the lovely explanation. Did you get the images off the internet ?? Its a really bizzare approach and hence my question.

Originally Posted by searchingheaven

No, these photos have been taken by me. But in my opinion, Dubai OMDB 12L is a very easy approach. There are lots of visual cues like the Burj Khalifa, the palm islands, the sea itself. And a long final.

Thanks!! I am fairly familier with the approach having flown there many times but it is just the set up that confused me. Let me explain :
1) The DH is set to 200 on Radio. I am asuming that this was an autoland in fairweather condition and hence the CAT 1 minima on a fail passive autoland system but why is the minima set on Radio? Does this operator recommend setting CAT 1 minima on Radio rather than Baro ??
2) At close to 500 feet the target approach speed is abou 13kts more than the Vref. What was the reason? I can see from the ND that the Ground speed is 136Kts, so it surely cannot be for wind compensation.
3) Was this a ferry flight ? I have no time on the type but the Vref seems extremely low for a 300ER.

[searchingheaven]

Originally Posted by RVD

Thanks!! I am fairly familier with the approach having flown there many times but it is just the set up that confused me. Let me explain :
1) The DH is set to 200 on Radio. I am assuming that this was an autoland in fairweather condition and hence the CAT 1 minima on a fail passive autoland system but why is the minima set on Radio? Does this operator recommend setting CAT 1 minima on Radio rather than Baro ??

2) At close to 500 feet the target approach speed is about 13kts more than the Vref. What was the reason? I can see from the ND that the Ground speed is 136Kts, so it surely cannot be for wind compensation.

3) Was this a ferry flight ? I have no time on the type but the Vref seems extremely low for a 300ER.

Nice to meet a fellow pilot.

To answer your questions:

1. Yes, generally the preference here is for a RADIO Decision Height since it is not dependent on you setting the correct QNH or pressure errors. However, if the DH is above 300 ft using BARO can be preferable since it is not subject to errors caused by unevenness of the terrain passing underneath the aircraft.

2. Initially, we had planned a flaps 30 landing in the FMC. But then decided to go with 25.

3. Yes, consider it somewhat like a ferry flight. There were just 20 passengers.

[Jeroen]

Originally Posted by searchingheaven

1. Yes, generally the preference here is for a RADIO Decision Height since it is not dependent on you setting the correct QNH or pressure errors. However, if the DH is above 300 ft using BARO can be preferable since it is not subject to errors caused by unevenness of the terrain passing underneath the aircraft.

.

This is something I often wondered about. I understand the basics of a radio/radar altimeter and how it measures it height. But how does it cope with everything on the ground. Buildings, forest, high rise, cars, trucks, fly overs, viaduct, all sort me of stuff below them for the last couple of miles that does not make for a smooth even surface at all!

From what I have seen these things give a very stable reading no matter what is below them?

How do they do that?

Jeroen

[RVD]

Originally Posted by Jeroen

This is something I often wondered about. I understand the basics of a radio/radar altimeter and how it measures it height. But how does it cope with everything on the ground. Buildings, forest, high rise, cars, trucks, fly overs, viaduct, all sort me of stuff below them for the last couple of miles that does not make for a smooth even surface at all!

From what I have seen these things give a very stable reading no matter what is below them?

How do they do that?

Jeroen

No, they do not give smooth reading irrespective of what is below. Case in point is Mangalore ILS 24. Since the airport is on top of a hill, the RA shows large value till almost over the threshold. That is presisely the reason why most operators prefer Baro minimums for upto a Cat 1 approach. Its different for airports that support Cat 2 or lower as they are required to have a clear area extending from the runway threshold in order to get accurate reading on the Radio Altimeter.

[vivtho]

I have to say that I'm a bit worried that the quality of TBHP's reviews is going down the hill. This is primarily due to several faults I've noticed in the review, some of which are

• How can you 'review' a vehicle without even getting into the driver's seat?
• There is no mention about the comfort or the passenger accommodations! How on earth are readers to understand how much space the rear passenger will have if the pilot pushes his seat back all the way?
• The reviewer simply ignored to mention how many 1L water bottles can be stored in the door compartments ... or even how many door compartments are there.
• What is the boot space?
• What is the build quality? Not mentioned. There is no information about panel gaps etc.
• No mention about the ground clearance of this thing
• Most of the review was spent in talking about the instrumentation. No pics of the vehicle itself!
• Most importantly ... kitna deti hai?

I have to say this is one of the poorest reviews I've seen on TBHP and so I'll have to rate it 1/10.

Hopefully people realize I'm being satirical.

[Fiddler]

[quote=Jeroen;3956794]Dont think so, it is simple not reliable enough when it comes to altitude.
]

Another way of explaining why it is not used.

Barometric Altimeter uses Ambient conditions to measure height.
Radio Altimeter RA transmits a beam downward and measures the reflection time and by simple formulae calculates height above ground.
GPS altimeter uses trigonometry from 6 satellites to determine height or location. This is in absolute value and is called true height it is from centre of the earth.

How do you seperate two AC from each other by 1000'?

1.) RA becomes unreliable because the terrain below AC A and AC B may not be the same.
2.) GPS Altitude though it might be correct. Both AC may not be receiving signals from the same satellites. No of variables are more.
Enter The Baro.
It is dependent on the atmospheric pressure at a location. It does not vary significantly over a wide area.
Two aircraft in a similar geographic location are separated in height by the atmospheric pressure. This atmospheric pressure is compared to An agreed standard called 1013.2hpa or assumed sea level pressure.
Since both AC are compared to the same datum, it is easier to ensure height separation. They are actually separated by pressure.
The only errors in this system are Instrument errors and location of instrument errors.
Hence each AC has three Pressure Altimeters with tolerance of +\-200' at 40000ft.

[Fiddler]

Originally Posted by Jeroen

This is something I often wondered about. I understand the basics of a radio/radar altimeter and how it measures it height. But how does it cope with everything on the ground. Buildings, forest, high rise, cars, trucks, fly overs, viaduct, all sort me of stuff below them for the last couple of miles that does not make for a smooth even surface at all!

From what I have seen these things give a very stable reading no matter what is below them?

How do they do that?

Question of height and speed.

The RA do measure it, but how wide are these buildings. By the time the reading is registered you would have already passed it and it is showing the next reading Unlike high terrain which can be wide spread.

[searchingheaven]

Originally Posted by Fiddler

The RA do measure it.
------_--------------------

In see from your profile page that you're rated on the A320. As you might have guessed, I am a Boeing guy through and through, but had an opportunity to use a A320 simulator recently. My question is quoted below from the original thread

Originally Posted by searchingheaven

I had the opportunity to use an A320 Simulator recently. The flight was uneventful, flying at 0.78 mach at FL300, in managed mode with a CORoute in the MCDU. There were a couple of problems though. The LVR CLB warning wouldn't go off even after reducing thrust lever to CLB detent. Also the FAULT light in BLUE Elect pump was popping up every now and then with warnings issued in the UPPER ECAM page as well.

As far as I understand, the fault light comes on if the reservoir level is low, or the reservoir overheats. Or the pump is delivering low pressure or is overheating. Same if the Reservoir Air Pressure is low. But I checked for all of these 5 parameters in the ECAM HYD page but couldn't find anything amiss.

Why was this happening?

[Fiddler]

Originally Posted by searchingheaven

I am a Boeing guy through and through, but had an opportunity to use a A320 simulator recently. My question is quoted below from the original thread

In the absence of Actual failure any ECAM should be considered Spurious.

To determine that

Case 1.) LVR CLB. N1 for CFM Engines. EPR for IAE.
The thrust rating and limit are indicated to you on upper ECAM.
They are based on TL position. CLB/MCT/FLEX/TOGA

If CLB is indicated in a Box along with EPR and Both engines are showing same EPR then you are in THRUST CLB.

If it is showing MCT or FLEX, or the above condition is met but the LVR CLB still flashes,

1.) One or both of the levers are possibly out of detent.
2.) The TL sensors in the Venus quadrant are indicating wrong position. Simulator problem.

Case 2.) Blue HYD Fault light.

Failure could be actual or spurious.
Cause of Fault illumination can be either reservoir based or Pump based.

Reservoir based.
OVHT
LOW LEVEL/LEAK
RESERVOIR LO AIR PRESSURE

PUMP Based. Blue pump is electrically driven.
PUMP FAILURE
PUMP OVHT
PUMP LO PRESS

If it is none of the above and the output is 3000psi +\- 200 then the fault can be considered spurious.

Another nice way of checking is in MCDU. If you go to CFDS on the MCDU you have three options while airborne.
1.) Current Leg ECAM caution/warnings
2.) Current Leg Faults

Option 2 will give you the failed item, if any.

[Jeroen]

Originally Posted by RVD

No, they do not give smooth reading irrespective of what is below. Case in point is Mangalore ILS 24. Since the airport is on top of a hill, the RA shows large value till almost over the threshold. That is presisely the reason why most operators prefer Baro minimums for upto a Cat 1 approach. Its different for airports that support Cat 2 or lower as they are required to have a clear area extending from the runway threshold in order to get accurate reading on the Radio Altimeter.

I don't know about such extreme, but I have seen them very stable flying over Cities, park, canals etc.

You are suggesting that when one would use DA over DH that would also be used to perform the auto land? I'm not so sure about that, as far as I know, on Boeings at least it will always use RA. The DH/DA is purely the read out on the PFD.

Jeroen