B737 Classic Runway Analysis max weights versus QRH speeds for light weights.

[Tee Emm]

We know that individual runway analysis charts list performance limit weights for a variety of flap settings, bleed configurations, airport temperatures and headwind and tailwind components. Worst case is probably based on 50 degrees C and say 10 knots tailwind (depending on chart design).

Occasionally the weight of the aircraft may be light and significantly less than the any of the weights listed on the analysis. Typically a ferry flight or some other event where the weight is well below average.

For the B737 Classics this is covered (or does this depend on personal interpretation?) by a QRH preamble at Chapter P1 Section 16 page P1.16.1 under the sub-heading Takeoff Speeds, that states: " The speeds presented in the Takeoff table, as well as FMC computed takeoff speeds, can be used for all performance conditions provided adjustments are made to V1 for clearway, stopway, brake deactivation, improved climb, contaminated runway situations, brake energy limits with unbalanced V1.

These speeds may be used for weights less than or equal to the performance limited weights.
............................................................ ..................................

My example: Lowest weight to be located on a runway analysis chart is (say) 52,000 kgs which is found at the 10 knot tailwind 50C temperature position on the runway analysis chart for flap 5. However, let us assume the actual aircraft weight is 42 tonnes or 10 tonnes less than the worst case performance limiting weight described above of 52 tonnes.

Am I correct in saying that in this case I can use the QRH speeds for the actual weight of 42 tonnes - based on the advice provided in the QRH? After all, V speeds are based on actual weight - not a fictitious weight for convenience.


If not, why not?

[G-Dawg]

I'm sure others will add, but I would go into the Performance section of the QRH get the V speeds from the Take off speeds section, gives weights from 30T-65T and use 50 degrees as my sel temp, which is our max..not sure if this helps at all......

[Old Smokey]

I've not done any Performance engineering work on the B737, thus my reply carries this caveat, but I can see no problem in operating as you suggest, i.e. using the QRH V speeds (with the required adjustments that you allude to), and an assumed temperature of 50°.

The QRH, Airport Analysis, and FMC speeds are all approved speeds. Using a thrust setting higher than required is a little akin to using the (assumed temp) thrust required for the Airport Analysis RTOW limit at, say, 10 knots headwind, but actually operating with a 20 knot headwind.

The reason that the airport analysis 'stops' at 50° is PROBABLY because that is the Environmental limit for the engine/s, i.e. ISA+35° at Sea Level. In some aircraft (e.g. B777), to address the case where actual weight is less than that available at the Environmental limit, the Airport Analysis continues beyond that limit (e.g. 60°), and data typically has an "A" suffix, where A = Above Environmental Limit. This does not give approval to operate in actual temperatures above the environmental limit, but to further reduce the thrust as though the actual temperature was above the limit. In this case the FMC speeds become invalid, and the Airport Analysis speeds must be used (FMC and QRH usually stop at the environmental limit).

Then again, the Airport Analysis may stop at 50° is simply because the RTOW provider has used a 'practical' range of Temperatures and Wind Components, to avoid using foolscap sized pages for the A/A book. The A/A RTOWs that I produce range from +10° to ISA+35°, to contain the data within practical limits. (If the actual temperature is <+10°, simply use the data for +10°. If it's above ISA+35°, don't fly).

That explanation may make sense, and then again it may not, I am not B737 qualified. To answer a question with a question, what is the Environmental Limit Temperature for your aircraft?

Regards,

Old Smokey

[mutt]

Interesting comments, for the B737's that I'm familiar with the envelope went to ISA +39.4C. In this part of the world, ISA+35 isnt sufficient, we just had the E170 recertified to ISA+39! IF his analysis stops at 50C, it sounds like an input limitation rather than a program limitation.


In this case the FMC speeds become invalid, and the Airport Analysis speeds must be used (FMC and QRH usually stop at the environmental limit).

We dont carry V-speeds nor are they published on the analysis, the FMC provides them right up to the maximum thrust reduction temperature which is generally about the environmental envelope.

Am I correct in saying that in this case I can use the QRH speeds for the actual weight of 42 tonnes - based on the advice provided in the QRH? After all, V speeds are based on actual weight - not a fictitious weight for convenience.

I would say that the answer is YES, you are correct. Using the FMC and runway analyis, we check for the highest assumed temperature and limiting weight. This temperature is entered into the FMC with the ACTUAL weight, V-speeds are for the actual weight and not the limiting weight (unless of course they are equal.)

Mutt

[Centaurus]

The subject of QRH speeds and personal opinions brings back memories. Many years back I worked for a 737-200 operator in the Pacific. One of the destinations was Tarawa. This was an atoll which saw a short costly battle between Japanese forces that held the atoll and its vital airstrip, and US marines who finally overwhelmed the defenders. Although Tarawa was the name of a series of atolls surrounding a lagoon, Betio was the atoll that held the airstrip. A larger airstrip was built after the war on Bonriki atoll some 10 miles from Betio.
Bonriki in 1979 was 6500 ft long and made of coral. The surface made for a bumpy, teeth rattling take off run. Ops decided that pilots should use full thrust for all takeoffs in order to reduce the the take off roll and thus exposure to severe vibration. The chief pilot refused to countenance flaps 15 or 25 in order to get airborne in a shorter distance - notwithstanding our take off weights allowed these flap settings. Worse still, he directed that the V speeds to be used for take off were for the performance limiting gross weights for the ambient temperature. As the OAT was invariably 30C, and flap was 5, it meant that for bleeds on full thrust the gross weight from the runway analysis chart gave a figure of max structural of 53 tonnes. Most times we were barely 42 tonnes.
So there we were roaring down the bumpy coral runway with frightened locals covering their ears from 2.15 EPR and near limit N1 and a VR of around 132 knots when with a modicum of commonsense the aircraft could have become airborne 20 knots less, if the speeds for the actual gross weight were used. The coral surface caused significant wear on the tyres - made worse by the higher than needed V speeds.
In my experience there are marked differences of opinion between pilots on the subject of V speeds. Some prefer to use lower flap settings in order to give better gradients of climb and in doing so accept the increased risk of tyre failure at high ground speeds or a decreased stopping margin associated with higher V speeds.
On the other hand, others argue for a greater flap setting within performance limitations in order to give lower V speeds and thus more margin for stopping before V1. My preference is for the latter - again assuming all performance limits are met.

[Old Smokey]

A further caveat that I should have added to my earlier post is that unless all of your data is for a balanced field, then whilst common sense dictates that a higher than required thrust (weight < max assumed temperature limit) should be acceptable, you may not have a legal leg to stand on if you are using UNBALANCED field data.

In the second paragraph of the original post it was mentioned in the preamble description that adjustments must be made for Clearway, Stopway, etc.

The FMC does NOT make allowance for unbalanced field.

The Airport Analysis DOES make allowance for unbalanced field (if used), and all data extracted from the A/A, including the Assumed Temperature, are derived from this.

Does your QRH provide the pilot means of deriving Assumed Temperature along with the corrections to V speeds for Clearway, Stopway, etc.?

As a general rule, you can't mix and match QRH, A/A, and FMC data unless you're using the balanced field principal. Beware the lawyers at the investigation!:eek:

I'd better bow out of this one, I'm not B737 qualified, but there's my 2 cents worth.

Regards,

Old Smokey

[Centaurus]

I think the original post was aimed at the situation where the actual weight of the aircraft is less than any of the performance limiting weights displayed on the runway analysis.

Already the unbalanced field length and other performance limitations that may be present are taken care of by the very nature of the runway analysis. For example if the actual weight of the aircraft is 42 tonnes and the lowest figure anywhere on the runway analysis is (say) 50 tonnes with appropriate V speeds tabulated for 50 tonnes, then it is logical to assume that at 42 tonnes the aircraft would perform well in excess of that experienced with a 50 tonne take off. This I believe is the rationale behind the QRH statement that the QRH speeds may be used at weights equal to OR below the runway analysis performance limiting weights. And as the analysis has already accounted for all the other factors (clearway, stopway etc) there is no need to make further adjustments to the QRH take off V speeds.

[BOAC]

Be wary of the Vr which SHOULD be higher at a higher than ambient assumed temperature due to less vertical thrust component. AFAIK the QRH speeds are for full power. The safest way (IMO) is to take the max assumed temperature available and use those speeds (assuming your weight is below that table weight, of course). You also avoid the pitfalls of ASD etc and book shuffling:). She will certainly rotate at that Vr, and V1 will be well on the safe side. All this assumes you are not taking off on coral, of course!