Landing Distance Calculations with Reversers 1

[aerodog]

Does any body know if Boeing factors in reverse thrust
when they calculate landing distances over the 50 ft
obstacle for the 737?

FAR Part 25.125(f) states you have to assume an engine
out condition upon landing so I always understood
reversers were never part of the stopping distance
equation.

Further that the main justification for reversers was
economic, savings on tires and brakes.

 

[CESSNA1]

AERODOG: Contact the FAA, Boeing, and other airplane manufactureres are required to demonstrate that the tires/brakes can stop the airplane without the use of thrust reversers.

Thrust reversers are much more efficient than brakes alone in slowng down the airplane that is why they are used. They generate about 80% of the forward engine thrust. brakes are used at the end of the landing roll.

Regards
Dave

 

[aerodog]

CESSNA1

So even with malfunctioning reversers, the 737 at
Chicago Midway should have been able to stop on
the runway assuming the pilot was flying otherwise
by the numbers, ie. at a 50 ft height over the end
of the runway, flying the approach at Vref plus
any adjustment for gusts, and increasing the Flight
Manual Landing Distance as necessary for snow/ice
on the runway.

Would that be your conclusion?

 

[CESSNA1]

AERODOG: We do not yet know what happened at Midway. Snow/ice/wind/airplane condition/etc all complicate the situation immeensly. I am not knowledgeable enough about the distances for a 737. From what I have observed as a passenger, pilot's usually apply thrust reversers first to slow the airplane down to a point where they can apply the brakes. On a snow/ice covered runway you never know what the real conditions are until the wreckage is cleared. I offer no conclusion except that perhaps, using 20-20 hindsight, Midway possibly should have been closed earlier. Airplanes are built for flight, not running on the ground, and certainly not negotiating snow/ice.

Regards
Dave

 

[davidjh]

The answer is complicated. It varies with the "type certification basis" which the manufacturers work to. The operators have to honor the requirements in (the US) FAR part 121. Older airplanes had to demonstrate a "landing distance", on a dry smooth runway, with brakes only (no reverse), as part of their "type cert". That distance was factored by 1.6 to create a "landing field length", which had to be honored by commercial operators. All manufacturers published "advisory data" which included the effect of reversers (all, and minus 1), and for a range of "non-dry" runway conditions. As time progressed the advisory data morphed into more of a regulatory requirement. The methods, quantity (and quality) varied greatly. The coefficient of friction (mu)of snow and/or ice covered runways varies greatly (with time, and along the runway length). There is a procedure to broadly rate the runway mu in these type of conditions and pass that on to the flight crews. Modern airplanes will have stopping distance data as a function of mu in the crew operating manuals as guidance info. Having said all that, the most dangerous problem is loss of directional control in those cirumstances. A lagging reverser or inop reverser will create a sideforce, which at the low runway mu's, may not be able to be counterbalanced by the tires and the rudder. There is a complex "force balance" diagram involved - Douglas published a good article on it in their operator newsletter some years back. Many people involved in analyzing this situation will say its better to go off the end of the runway than off the side.

 

[aviat]

The simple answer to your question is yes; the airplane should have been able to stop without thrust reversers. But it’s not that simple, thrust reversers may be involved in the cause of the accident. This is really an operations issue because the airplane is already certified.

Very basically, for dispatch purposes, thrust reverse cannot be included in the stopping distance, as well as only one half of head wind and 150% of tail wind component. There are other items involved such as designation airport runway slope (uphill or downhill), pressure and temperature different from standard at intended time of landing, as well as intended landing weight. From this information you must be able to land in 60% of your designation airport’s runway, as per FAR 121.195 for dry runway, or 115% for wet runway. FAR 121.195 are for dispatch only; if things change in flight, you can use 100% of runway if you have to. Note: AC 121.195 explains certification requirements for wet runways.

That is how it works most of the time, but there are times when things line up to cause problems. The book Fly the Wing lists some rule of thumbs that show differences from an ideal, dry runway:
Antiskid inoperative – add 75% from max antiskid breaking,
Ground spoiler inoperative – add 25%,
Reversers inoperative– add 10-20 %.
If you cross threshold at 100 feet instead of 50 – add 900 feet to touchdown distance.
1% increases in speed over correct V(ref) – add 2% change in landing roll.
1% increases from max landing weight - add 1% change in landing roll.
Add other factors like ATC, maintenance problems etc. that may be involved.


Most accident of this nature have several factors involved, both active and latent. There is usually no single cause. After an accident the media runs around trying to find “the cause” or someone to blame. Then they lose interest and move onto something else. It is when some of the factors above and others, line up, with no defences that accidents occur.

The following site is from Transport Canada’s site and shows that when landing on ice, the stopping distance can increase substantially without reversers as was discussed in previous post.

http://www.tc.gc.ca/CivilAviation/commerce/OperationalStandards/CRFI/menu.htm

The following site has a calculator for the B737 landing distance.

http://www.b737.org.uk/landing_distance.xls