How regular is the rumble?

[Scarbagjack]

Hi guys, I have just started flying fairly regularly again after 20 odd years. About 24 flights per year for the last couple of years mostly domestic to the Australian Capitols and from there regional. I have done a couple of long hauls recently as well, BNE-LAX, and have a couple of questions that I hope will not sound too dumb. Firstly, whilst flying on the BNE-LAX pacific skyway, after some amount of time I would notice the engines increasing in thrust for a while, I am assuming that as more fuel burns off the higher we want to get to get on board with the jet streams? Is this right?
Also on one flight as we began our decent into LA the spoilers were deployed which sent the 747-400 into a lovely state of rumbling vibration. Now I was fortunate enough to be invited onto the flight deck at the completion of that flight and asked the boss about this, he replied that coming into LAX requires a high and quick decent to comply with some US flight regulations, and sometimes they need the airbrakes to help with this. I dont doubt the validity of what he was telling me, but I have never been privy to this before or since that particular flight. The speed was showing 1009 kph on the seat back screen just before the decent. Is it possible for these babies to " get away on you " when riding in a well charged jet stream? How regular does this happen.
I am a big fan of the 747-400s, I reckon they are as sexy to look at as VE commodores and Haines Hunter boats ( just my personal opinion ), but after the rumble experience, I was still shaking long after the big bird had stopped
Enough for now, a bit long winded I know, but would appreciate any sharing of experience on these things.

Cheers

[Hartington]

In my experience as SLF airbrakes are a regular occurrence, particularly on approach to US Airports. At San Francisco the approach was described to me by a pilot as often being cross the airport going down the bay towards San Jose at 12000 ft and you'd better not be doing more than 200 kts at that point because you then have a track of 20/25 miles to lose that 12000 ft and the speed before crossing the threshold. You just go down the bay to somewhere near Redwood City before turning through 180 degrees to land on on of the 28s.

I was also told that when BA bought A319s they specified the option of airbrakes and this particular pilot was glad they did.

[B314]

Not quite in the same vein but I recall a flight from LIS - LHR in 1969 and the BEA Comet I was on seemed to have a much steeper angle of climb than 707's or DC8's I'd been on previously.

[Intruder]

Controllers at LAX often keep airplanes too high for too long while dealing with lower-altitude traffic. If the pilot (and the FMC) has planned for an "optimum" VNAV descent, any delay in the descent will require airbrakes or early flap and/or landing gear extension. The rumble is most noticeable in the 744 only with the last inch or 2 of airbrake lever travel, so many pilots will judiciously use it. Some times, though, we just have to pull it all the way back...

[Scarbagjack]

Thanks for the replys, so I suppose I can take it that the speed brake applications,relating to higher/quicker type of descents, occur mostly in the US..or when other factors warrant their use.

I dont know much about flying aircraft, so please bear with my queries, but can an aircraft get away on the piolt in an agressive jetstream, as in excessive speed? If so, what actions will they take to bring the speed back..such as descending below, ( with ATC clearance), or out of the fast winds,throttling back, or spoiler activation.

I mean in the context that the aircraft is not ready for its planned descent at this stage.

Thanks again.

[Sultan Ismail]

Speed through air versus Speed across the ground

Scarbagjack, the speed you are seeing is the ground speed and is quite normal when in a jet stream. I have regularly flown from Zurich to Bangkok at 38,000 feet with an indicated speed of 1111 kph, quite exhilarating.

If you tried to measure the speed through the air it would probably be 250 kph (that's not a fact) as it is measured against local atmospheric air pressure and hence we end up with actual speed which is measured in Mach, and there your 744 would be rolling on at M0.84.

[Scarbagjack]

Sultan, thanks for your reply. Yep it sure does feel exhilarating when one reads the speed indications on the tracking map on the screen.
Makes sense what you are saying..I have never experienced the air brake application in flight on any domestic flight here in Australia. I think the pilots conduct longer slower descents, not sure the reason why, but I do remember somebody mentioning fuel savings by flying in this manner.

Thanks again.

[Northbeach]

shake rattle rumble and roll..............not to worry.

Scarbagjack,

Los Angeles is a busy place, four runways two for departure and two for arrivals. There are airplanes coming in from all four cardinal directions. From the West the Pacific traffic from the Hawaiian Islands, New Zealand and Australia. From the north domestic traffic from San Francisco, central California, the Pacific Northwest and Alaska/Asia. The south brings in our “compadres” from Central and South America as well as the traffic from Mexico. From the East come the arrivals from central/eastern Canada, Europe and the Eastern Seaboard of the U.S.A.

From LAX dozens of carriers and freight operators launch everything from piston-powered twins to the newest 380s going to virtually every corner of the globe. In close proximity to LAX are several other major airports (Long Beach, Orange County, Ontario and Burbank). Throw into this mix a healthy number of Business/Corporate jets and many general aviation airplanes and you have congested airspace.

A single flight arriving into Los Angeles does not have the luxury of having an uninterrupted descent (unless you happen to be some VIP head of state getting special treatment). Often air traffic controllers change the parameters by assigning a different speed or altitude several times during a descent. Or they are concerned about maintaining the required separation between airplanes they are controlling. Sometimes they will ask the pilots to expedite their descents. And then there are times I do not calculate correctly my rate of descent and need to take additional action to get my airplane where it is supposed to be in the descent.

One of the tools at my disposal are the “speed brakes”. The speed brakes are metal panels on the topside of the wings close to the body of the jet towards the back part of the wing. They are normally held close to the wing. However by using a lever I can direct hydraulic pressure out to the speed brakes and make them begin to stand up away from the wing and into the air passing over the wing. When I move the handle and extend the speed brakes it makes the wing less efficient in producing lift and at the same time causes the air passing over the wing to become much more turbulent. You will hear a rumble and feel the shake as the air passing over the wing is disturbed by the extended speed brake.
The faster I am flying the more you will hear and feel the effect of the speed brakes. Their extension adds much “drag”, resulting in a greater rate of descent.

There is nothing wrong with your wing, pilot or flight. The manufacturer built in this capacity and gave your pilot the speed brakes as an additional tool to be used to manage their descent rate and speed. There is no reason to become anxious when you notice the speed brakes being deployed, your pilot has decided to increase their rate of descent and/or reduce the forward airspeed of your jet to get your jet to a certain point in the air at an assigned altitude and airspeed.

When your pilots add power to the engines to begin their takeoff roll you also have a much-increased noise level and vibration. That probably does not concern you as it is to be expected. A similar thing is happening on descent the noise and vibration is increasing, but this time it is the shape of the wing causing the rumbling rather than the amount of power being demanded of the engines. There is no reason for you to be concerned at all. Because you understand what is going on you can now sit back relax and enjoy the experience. Once your pilot has achieved what they needed to do they will once again “stow” the speed brake and you will notice both the vibration and noise level decrease.

Respectfully,

Northbeach

[Scarbagjack]

Northbeach, thanks so much for the detail!!
I love visiting the US, will be back in August, but the biggest difference, I suppose, between our two countries is the size of the population.
The amount of air traffic in and around LAX must, of course, be quite significant.
When I visit this time I will be taking my first domestic flight within the US,
LAV-JFK. I imagine the same sort of scenario might unfold on your internal flights as well.
I shall most certainly keep that info in mind, my pleasurable flying experiences involve trying to understand what the pilot and/or aircraft may be up to,therefore, I enjoy research and checking things out.
My wife,however, maintains its not her job and somebody will get us there safely. We both travel happily.

Much appreciated, cheers

[Intruder]

I suppose I can take it that the speed brake applications,relating to higher/quicker type of descents, occur mostly in the US..or when other factors warrant their use.

No, they don't occur mostly in the US, unless you assume that most flights occur in the US. There are many places around the globe where ATC occasionally requires steeper than desired descents.

[Northbeach]

When and why we climb to higher altitudes.

"after some amount of time I would notice the engines increasing in thrust for a while, I am assuming that as more fuel burns off the higher we want to get to get on board with the jet streams? Is this right?"




The higher up into the atmosphere one goes the less dense the air. The less dense the air the less amount of drag is produced by moving a body through that “thinner high altitude” air. At 18,000’ above sea level one will find approximately half the atmosphere density found at sea level.

Given the decrease in drag resulting from the much less dense atmosphere found at higher altitudes it becomes more economical to fly at the higher altitudes; the engines burn less fuel to move the body of the jet through the air. That aircraft’s mass is generating less drag, because of the less dense air offering less resistance to its movement.

The difference is significant. Your long haul widebody carries ample fuel to make the flight from distant Australia or Europe to LAX at the higher altitudes high 30,000’ to low 40,000s above sea level (10,000-13,000 meters). That same jet most likely could not make the identical journey at 10,000’ (3,000 +/- meters). It would run out of gas long before arriving as it couldn’t possible carry enough fuel to fly the entire distance at very low altitudes. Because flying low that aircraft, in the dense atmosphere, the jet engines burn far too much fuel to make them an economical source of power. Up high in the tropopause the high bypass turbojet fan engines are in their element being able to produce enough thrust out of the thin air to overcome drag while “sipping fuel” (relatively speaking) as they drive the jet through the atmosphere.

So you are correct. Your flight starts out at the most economical altitude for that day given the weight of the aircraft, ATC (Air Traffic Control) requirements, and weather. As your jet burns off tons of fuel it becomes lighter, making flight into higher regions of the atmosphere (greater efficiency) possible. Usually your crew wants to fly higher unless there are reasons not to do that on a given day; adverse winds, turbulence aloft, high/low speed buffet margins, MEL (mechanical status) considerations and a few others.

We have some very smart computers on board, charts the manufacturer provides us plus decades of experience that provides us with guidance as to when we should initiate a climb and what altitude will result in the greatest fuel savings for kilo/pound/ton of fuel consumed for distance traveled given the weight of the jet given the winds and temperatures at altitude on that day.

Climbing higher to ride the tailwinds of a jetstream may be part of the reason to initiate the climb, as long as all the other necessary parameters are met.

Pleasant journeys………….

[Seat62K]

A question which, I'm sure, has been asked many times before:

Is it more fuel-efficient to remain at cruise altitude as long as possible, using spoilers to descend rapidly; or is it better (i.e., more fuel-efficient) to reduce thrust and let the aircraft descend much more slowly from farther out?

Any answers?

Thanks.

[Intruder]

Minimum drag is best for most airplanes, so top of descent is planned based on no speedbrakes.

OTOH, I flew one jet airplane (US Navy T-2C Buckeye) that burned so little fuel at altitude that the emergency profile included a speedbrakes-out descent. It also had a straight wing, so that probably had a lot to do with it...

[Scarbagjack]

"We have some very smart computers on board, charts the manufacturer provides us plus decades of experience that provides us with guidance as to when we should initiate a climb and what altitude will result in the greatest fuel savings for kilo/pound/ton of fuel consumed for distance traveled given the weight of the jet given the winds and temperatures at altitude on that day."


Thanks again Northbeach, so in reference to the above, does the computer itself actually commence the climb without any input from the crew?, or does the pilot in control at the time take over when prompted by the computer and manually adjust the aircraft settings for climb?

Thanks

[Winglet__]

Great Question....

The on board systems will let the pilots know when they think a 'step climb' is recommended. It bases this on the weight of the aircraft, and speed schedule that the pilots have programed.

But onboard computers are only as good as the programmer that wrote their system, and the information that they have been subsequently given.

So if the pilots are aware of turbulence at a higher level, or unfavorable winds at a higher level, or there are traffic or airspace issues, then they will manage the flight as required. And climb or descend as appropriate.

At no time is the aircraft able to climb or descend without deliberate pilot input.

This includes aircraft conducting an 'autoland'. The aircraft is not able to deploy the wing flaps, or the landing gear, without the direct input and judgement of the pilots.

Keep the great questions coming!

[Northbeach]

Great questions!

Scarbagjack,

Winglet is right on both counts. You ask great questions. Your starting post on this thread had at least 3 of them; speedbrakes, climb profile and jet upset. And Winglet is also correct stating the jet should not commence any climb without direct input from the crew. Winglet makes an important point about the computer only being as good as the programmer writing the system, and I will add, the person inputting the data. Garbage in garbage out and that usually isn’t the programmer or manufacturer’s fault. Unfortunately there have been several recent incidents where pilots have entered erroneous data with bad results.

I fly classic and NG 737s other jets will be different. On our airplane the flight management system (FMS) will prompt us (pilots) with a message on a screen in response to information I previously entered regarding final cruise altitude. On the NG I will look at the AOA (angle of attack indicator, shows me the actual performance of my wing) to back up the climb performance predicted by the FMS. If the ride is smooth, winds are favorable and ATC can accommodate my request for a higher altitude we will climb.

There is more than one way to climb. As you mentioned you will hear us demand more power from the engines to maintain the desirable speed as we climb. Once climb power is reached by the engines and we are “on speed” we will gently “pitch up” (raise the nose of the jet up into a climb) and depart the previously assigned altitude for the new flight level (Altitudes above 18,000’ are referred to flight levels; 35,000’ feet above sea level is called “Flight Level three five zero FL350 not three fifty.

I am going flying for the next few days. Keep the great questions coming! If you are wondering about something you can wager many others have the same questions but did not take the time to ask. There are many seasoned professionals here who are both knowledgeable and gifted communicators that will answer your questions.

And finally, thank you for buying the ticket and keeping all of us employed!

Respectfully,

Northbeach

[Scarbagjack]

Winglet makes an important point about the computer only being as good as the programmer writing the system, and I will add, the person inputting the data. Garbage in garbage out and that usually isn’t the programmer or manufacturer’s fault. Unfortunately there have been several recent incidents where pilots have entered erroneous data with bad results.

So true, in my line of work this also applies.and also, unfortunatley, that too can have serious consequences.

Just one more quick querie on spoilers.
Winglet mentioned autoland settings, I have heard that the spoliers can be armed to deploy on touchdown. Does this happen when the landing gear contact the runway?
And is reverse thrust actioned automaticly or only through direct control?

Thanks for all of the excellent replies, I have really been enlightened.

[Intruder]

On the 747 spoilers deploy on landing:
With weight on wheels and throttles 1 & 3 at IDLE, if the spoiler lever is in the ARMED position.

When reverser 2 or 4 is deployed, if in the DOWN position.

[Old Fella]

Jet Streams

Scarbagjack. Your question regarding the aircraft "getting away" is fair enough for a non-airman to ask. The short answer is No. The speed the aircraft knows is Airspeed. The speed over the surface at any normal cruise speed, e.g. Mach .84 typically in a B747, is governed by the winds at whatever level the aircraft is being flown. The speed over the surface "with the wind" will be the normal cruise speed plus the wind speed. The opposite is the case flying into the wind. Pilots and/or their on-board navigation system will determine the most appropriate Top of Descent (TOD) point so as to minimise fuel usage and meet any ATC requirement. In my RAAF days we used to occasionally conduct minimum fuel descents, which were more designed for fighters arriving back to base with low fuel states, in the C130 whereby we would home to overhead the airfield and essentially spiral down at flight idle. In my experience the use of speed brakes on descent is more the exception than the rule, not least of all because of passenger considerations. The same "speed brakes" are used in conjunction with ailerons to provide better roll capability in cruise where on many aircraft the outboard ailerons are "locked out" when flaps are up. Keep flying, it's the only way to go.

[Selfloading]

I love it when the pilots deploy the speedbrakes, not sure why just enjoy the rumbling and shuddering