Fuel dump or overweight return?

[skelsey 765]

12 hours ago, rondon9898 said:

Actually, Simon, I got a bit confused in my post sorry - I meant there was a system overheat, as I'm aware a failure of a demand pump wouldn't be critical. Would your reply change in that case?

No worries. I don't think it would -- again, the only advantage of an overweight landing is that it gets you on the ground quicker, but at the expense of more stress on the airframe than a landing below MLW and at an increased risk of hot brakes, runway excursion etc etc. Is it worth it?

It's not quite the same, but in a sense you could liken it to a high-speed RTO. The aircraft is certified for it, your performance calculations tell you that you can stop safely at any speed up to V1, but even so the stats show that high speed RTOs are inherently risky and frequently result in runway excursions and other problems. Thus, the takeoff roll is divided in to a low-speed portion where you consider stopping for anything, and a high-speed portion above 80 knots where you only stop for a limited number of certain clearly defined reasons - i.e. not because it is possible to stop, but because the aeroplane will not fly. This is because apart from these few critical scenarios, the risk of stopping is greater than the risk involved in solving the problem in the air.

You could look at an overweight landing in the same way. Is the risk of an overweight landing with all that extra energy (remember: E = 1/2mv^2 and in an overweight landing you are making both m and v significantly bigger) greater or less than the risk of staying airborne for an extra hour or so?

In almost all circumstances, I would generally suggest that it is going to be better to dump the fuel and land below MLW.

[rondon9898 84]

Got you thank you very much for those responses Simon - gives me a bit more context to work with in realistic decision making in the sim

[berts 97]

John,

Assuming you are already airborne, one of the first actions you must take when dealing with an overheat in one of the hydraulic systems is select a serviceable autopilot (Hyds 1,2 3, control A/Ps C,R,L respectively) .  The QRH actions should be carried out methodically and slowly because it can take a little while for the HYD OVHT SYS message to go out if the Demand Pump and/or the Engine Pump is at fault. If you end up switching both of these pumps OFF on the affected system then the QRH instructs you to initially ignore the HYD PRESS SYS message at this stage (for obvious reasons).  Please note that the B744 QRH definitely does NOT require you to dump or land at the nearest sutiable airfiled in this situation!

The aircraft is capable of flying perfectly well with three hydraulic systems and line crews are regularly trained and tested on their ability to fly with only two. Therefore, there is no need to panic in this situation because you have only lost one of your four hydraulic systems.  Unless there is another more serious problem (such as an uncontrollable cargo/fuselage fire) why would you want to dump fuel or divert from your original flight plan?  An uncontrollable fuselage fire is of course one of the worst things to experience in flight and I would certainly aim to get the aircraft back on the ground as quickly and as safely as possible in this condition - and I definitely wouldn't waste time or dump fuel with the aircraft on fire because it takes over an hour to dump from MTOW to MLW on a B744.  However, the fault in the hydraulic system might have been caused by a simple leak and loss which has eventually resulted in the overheat, in which case there might be a sufficient amount of fluid left after completing the QRH to allow you to turn the pumps on momentarily to lower the flaps and/or gear by the normal system prior to landing at your destination.

You should always plan for the worst case scenario and (as Simon has pointed out) when time permits DODARing the problem is always recommended. This should always include thinking about and planning for the approach, landing and possible go-around and diversion.  Carrying out the Hydraulic Overheat System QRH procedure may result in you having to carry out the HYD PRESS SYS checklist.  Even if you haven't totally lost the hydraulic system it is always sensible to have a look at what effect this will have on the operation if you subsequently lose it later on in the flight.  ATC will need to know and they are there to help you.  If the weather is below CAT 1 you will have to divert because CAT 2 & 3 autolands are not approved with any loss of a single hydraulic system.  A loss of hydraulic systems 1 and/or 4 have the biggest impact on the operation and you will also need to consider such things as the operation in icing conditions, the length of the runway, stopping distance, cabin pressurisation, spoiler operation etc  The list goes on!

Happy landings if you try this as a scenario!

Bertie Goddard       

  

    

  

 

 

     

[richjb2 195]

John,

From a certification perspective, any part 25 airplane must be structurally capable of safe landing at maximum certificated takeoff weight. I believe the assume touchdown rate is 6 FPS or 480 FPM at MCTW, but I would need to check part 25 just to be sure. Any part 25 airplane that cannot meets its OEI approach and AEO landing climb limit weight 15 minutes after takeoff from any airport within its permitted operating takeoff airport envelope must have fuel dump system.  However, it is not required that the pilot dump fuel in an emergency to bring the airplane down to its maximum weight where OEI approach and AEO landing climb are met .  That's a Captain's discretion item.

On a four-engine airplane, the PIC has the option to continue the flight. It's an old, somewhat archaic rule that dates back to the Constellation and DC-7 days where engine failure were quite common.  A few years back a British Airways Captain got into hot water with the FAA after they experienced and engine failure on takeoff during climb out of LAX and elected, legally, to continue to EGLL on three engines. The FAA conceded that the rules allowed the pilot to do that, but also asked BA to ensure that their crews didn't do that again in US airspace as it was always intended for oceanic operations particularly departing from remote airports.  There's a part 91 rule that requires the PIC to discontinue the flight anytime a mechanical problem affecting airworthiness, but the other rule regarding an engine failure on a four engine airplane created a grey area.

Notwithstanding an emergency, the PIC must ensure that the airplane is below maximum certificated landing weight in the event of a landing at an airport other than the planned destination (e.g., return to departure airport). For example, if the IRUs were not properly aligned prior to takeoff (no GPS) and there was need to return to the departure airport to re-align the IRUs, then fuel dumping to the MCLW would be legally required.  If the airworthiness of the aircraft is in question, then fuel dumping is not necessary as the PIC is using their emergency authority.  The QRH usually uses terminology like "Land as soon as possible" or "land at nearest suitable airport" to assist the PIC in assessing the need to terminate the flight using this authority. Some QRHs use the term "land as soon practical" to give the pilot greater leeway in making a decision to land.

Outside of performance considerations provided in the AFM or QRH, overweight landings are generally a maintenance function. Inspections will vary depending on the rate of touchdown. if greater than 3 FPS/360 FPM, then the inspection becomes greater.  Most modern airplanes have downloadable information that provides both the weight at touchdown and the touchdown rate.

Hope that this helped,

Rich Boll

Wichita KS

[MattS 45]

On April 24, 2017 at 9:10 AM, scandinavian13 said:

Granted, this wasn't an overweight landing, but the gear can actually take quite a beating, as this poor Silkway pilot showed us recently:

 

Kyle, equivalent flap shock animations would be greatly appreciated in the next update. Hahaha

On a serious note for those with RW knowledge, are mandatory inspections required and what do they involve following a landing such as this? How long would these keep the aircraft out of service?

[rsrandazzo 6,353]

John, et al,

In the sim community- folks have traditionally misunderstood what your MLW really means.

If you land at a weight that exceeds your MLW, It does NOT mean that the airplane will break.  To the contrary- airplanes touch down over MLW with relatively little fanfare.

The MLW is a weight that is established by the manufacturer's engineers as the weight at which the structure of the airplane will continue to lead a normal structural lifespan.

If you bought yourself a 777, and landed it 100,000lbs over MLW every single time you flew, that airplane would theoretically show signs of fatigue wearing at a rate faster than an exactly identical 777 that was routinely landed at a lesser weight.

Makes sense, right?

So from my vantage point on the flight deck- if I take off at 877,000lbs and and have a catastrophic failure that calls into question the safety of the airplane or it's occupants- I am going to make the decision to eat some of the stress life of the airplane by putting it back onto nearest, most suitable runway available.

If, on the other hand, the biggest risk to the flight is that I might inconvenience my passengers schedule, I will take the time to find someplace reasonable to dump fuel and reduce weight.

There are even occasions where the airline might decide it is necessary for me to return to the field- and they will make the decision to authorize an overweight landing pending the crew's agreement...  it all takes place within the sphere of the modern airline operation.

What happens after a landing over MLW?  The manufacturer has provided the airline with a detailed inspection to put the airplane through, depending upon the circumstances.

All in a day's work.

 

[77west 139]

As Robert points out, its not gonna break. Some airplanes actually have different MLW options the airline can purchase, with the higher MLW options costing more in both dollars and airframe life.

Freighters often have a much higher MLW to carry more payload with less range (trading fuel for cargo) and are also operated on a less intense schedule than most passenger frames, evening out the increased wear, somewhat.

Point is, in a fire, or serious emergency, get it on the ground ASAP