Minimum Weight Question

[1970s Spotter]

Can anyone carify what this might mean?:
I have a very old document relating to a ferry flight for a Boeing 707, it states "gross weight is minimum required for flight". Do I take it that this means that the a/c was to be ferried empty and with only the required amount of fuel?

Thanks

[john_tullamarine]

Difficult to say without having the relevant documents to hand.

However, some aircraft will have a minimum gross weight indicated on the CG diagram. Generally, this will be associated with Vmca determinations. It is a limitation although usually not a concern as the weight will be sufficiently low not to present a problem for the great bulk of operations.

[westhawk]

§25.25 Weight limits.



(b) Minimum weight. The minimum weight (the lowest weight at which compliance with each applicable requirement of this part is shown) must be established so that it is not less than—
(1) The lowest weight selected by the applicant;
(2) The design minimum weight (the lowest weight at which compliance with each structural loading condition of this part is shown); or
(3) The lowest weight at which compliance with each applicable flight requirement is shown.
What JT says seems reasonable to me as per 25.25 (b)(3) above. During my initial type rating training on the old Westwind 1124 I recall one of the instructors talking about how they were ferried to the USA from Israel "green" with no interior, minimal equipment installed and just enough lead ballast secured to the floor up forward to bring it up to minimum flight weight and within allowable CG at the end of the usable fuel supply. A typically equipped 1124 with a "corporate" pax interior is usually has a BOW about 1,000 lbs above the minimum flight weight. A few of this model were used for freight hauling and in a cargo configuration, could actually reach a weight below minimum if flown empty with very little fuel. I'm just guessing here, but I suspect the 1124 minimum flight weight was the lowest weight selected by the applicant or was simply as light as it was ever flown in testing.

[OverRun]

j_t, westhawk and Mad (Flt) Scientist covered the background to minimum gross weight back in 2006 :) http://www.pprune.org/tech-log/245955-minimum-zero-fuel-weight.html

[A Squared]

Do I take it that this means that the a/c was to be ferried empty and with only the required amount of fuel?

In absence of additional context indicating otherwise, that's how I'd interpret that sentence. Airplane should be as light as possible without running out of fuel prior to landing.

[Mad (Flt) Scientist]

As JT says, context is key here.

I can imagine that sentence really meaning "gross weight [as defined above] is minimum required for flight" - meaning that the document defines a weight, and is saying you mustn't go below it - which could speak to VMC or other considerations.

I can also imagine it really meaning "gross weight is [to be the]minimum required for flight" i.e. keep the aircraft as light as you can, with no lower limit.

The nature of the ferry might give a clue; if it was a ferry with significant airframe or systems damage (to get to a repair base) that might yield clues.

In our case, we've had some ferry flight permits where the aircraft had to be kept as light as possible, to minimize loads for example, and some where a minimum flight weight had to be achieved to respect VMC in light of system defects.

[JammedStab]

The 747-400 has a minimum weight. In my experience, the basic weight for the pax aircraft with its heavy furnishings is higher than the minimum weight so no issue anyways. But on the freighter, it is less than the basic weight. Mind you not by very much so it still should not be an issue. Maybe it could be for an aircraft stripped down to bare bones and then being ferried to its final resting place.

361,200 pounds minimum weight for the 747 freighter.

[ThreeThreeMike]

This brings to mind something I read in an accident report concerning the loss of a DC-8 freighter during takeoff for a three engine ferry flight. The crew used incorrect V speeds and improper application of throttles on the takeoff roll.

The report contains the statement:

The captain and his crew departed DOV on the first ferry flight and arrived in MCI at 1739 on the day of the accident. The block-to-block time for the flight was 3.3 hours.

AT1 arranged for a qualified airframe and powerplant (A&P) mechanic to fly from DEN to MCI to prepare N782AL for the three-engine ferry. The captain prepared the flight departure papers and discussed fueling requirements with another AT1 captain who had flown N782AL to MCI.

Both captains agreed that the fuel load should be 75,000 pounds, to include 30,000 pounds of ballast fuel and 45,000 pounds of usable fuel.

http://www.airdisaster.com/reports/ntsb/AAR95-06.pdf

I thought the decision to carry that much additional fuel was odd, given the fact the aircraft was being operated with three engines. The additional weight required higher V speeds and would have exacerbated controllability issues.

Not having any experience in the DC-8 (or any other four engine transport category aircraft) I am not familiar with minimum weight requirements and their effect on aircraft operation.

The above linked thread

http://www.pprune.org/tech-log/245955-minimum-zero-fuel-weight.html

gave some insight, but it just seems counterintuitive that an aircraft operating with degraded performance would be required have an additional 30,000 lbs of fuel on board.

Any comments would be appreciated.

[Mad (Flt) Scientist]

....The additional weight required higher V speeds and would have exacerbated controllability issues

.....

gave some insight, but it just seems counterintuitive that an aircraft operating with degraded performance would be required have an additional 30,000 lbs of fuel on board.

Higher speeds, yes, but the extra weight would have likely improved not degraded the controllability - if it was being carried to meet a min flight weight driven by VMCA, for example, it would end up being easier to fly with an engine out, which is what they were doing.

If they were having to add ballast fuel, they were nowhere near a performance limit in terms of heavy weight; they'd still have had to meet appropriate climb and takeoff distance etc.

Also, an excessively light a/c might have "too much" performance, in the sense of requiring excessive or unusual pitch attitudes to maintain the scheduled speeds. Going outside of the weight/cg envelope in ANY direction - heavier, lighter, more aft or more forward - all have their problems. The a/c was certified to be inside that envelope, so anything that keeps you inside the envelope is good.

[latetonite]

I am still not convinced about the explanation of minimum weight. Provided you take VMCA somehow in to account.

Ballast weight or ballast fuel is usually there for CG requirements.

On the other hand, the lighter the weight, the more G's will be there in turbulence. Maybe the engine pylons on the DC8 were the reason for this?

[FlyingStone]

IIRC it has to do something with Va being much lower than at standard weights...

[JammedStab]

I thought the decision to carry that much additional fuel was odd, given the fact the aircraft was being operated with three engines. The additional weight required higher V speeds and would have exacerbated controllability issues.

Not having any experience in the DC-8 (or any other four engine transport category aircraft) I am not familiar with minimum weight requirements and their effect on aircraft operation.

it just seems counterintuitive that an aircraft operating with degraded performance would be required have an additional 30,000 lbs of fuel on board.

Any comments would be appreciated.

Could it have been for balance reasons?
Have seen ballast fuel options on empty 727's and non-ideally loaded 747 combi.

[No Fly Zone]

I'm not an engineer, but try this... A Massive Wing Area (think feather?) and very little weight. Without some greater weight, ambient winds and other weather will control that 'feather' no matter how much thrust is applied. The control surfaces can do only so much, especially at low speeds. While 'forcing' a super-light 747 off the ground may be possible, controlling an even lighter one for landing in anything but a perfect headwind must be a significant challenge. I don't know how to fly feathers.:sad:

[JammedStab]

FAR 25.25 Weight limits.

(b) Minimum weight. The minimum weight (the lowest weight at which compliance with each applicable requirement of this part is shown) must be established so that it is not less than—

(1) The lowest weight selected by the applicant;

(2) The design minimum weight (the lowest weight at which compliance with each structural loading condition of this part is shown); or

(3) The lowest weight at which compliance with each applicable flight requirement is shown.

[JammedStab]

FAR 25.25 Weight limits.

(b) Minimum weight. The minimum weight (the lowest weight at which compliance with each applicable requirement of this part is shown) must be established so that it is not less than—

(1) The lowest weight selected by the applicant;

(2) The design minimum weight (the lowest weight at which compliance with each structural loading condition of this part is shown); or

(3) The lowest weight at which compliance with each applicable flight requirement is shown.

This Boeing document titled "Introduction to Weight and Balance" has this statement.

"Minimum Inflight Weight (MFW):
The minimum allowed weight during flight. Establishes the load factor for airplane fixed equipment"

Not really sure what it means though.

http://www.scribd.com/doc/133403992/Boeing-Weight-Balance

[Mad (Flt) Scientist]

For major items of structure, the loads are a function of the overall aero forces, or the load factor times the weight (mass). So the loads on the wing structure, for example, are similar at 50,000 lbs and 3g as at 75,000lbs and 2g. But a piece of fixed equipment - say a radio - sees a load on its fittings dependent on its own mass and the g. So the highest loads on things like brackets for radios occurs at min weight, as that's whee the highest g is created.

Of course, it's never that simple.