B737's. Releasing the park brake with chocks in place
After landing and when the aircraft has taxied to the terminal, it has always been Boeing policy for the parking brake to be released once the wheel chocks are in place. This was common policy for many aircraft types and was a precaution against setting the parking brake against hot brakes.
The current FCTM makes this point under the heading of Taxi Speed and Braking:
"High taxi speed combined with heavy gross weight and a long taxi distance can result in side wall overheating. Avoid prolonged brake application to control taxi speed as this causes high brake temperatures and increased wear of brakes. If taxi speed is too high reduce speed with a steady brake application and then release the brakes to allow them to cool. Braking to approximately 10 knots and subsequent release of brakes results in less heatbuild-up in the tires and brakes than when the brakes are constantly applied". Notice the words, "release the brakes to allow them to cool".
The shut down procedure in the current B737 series FCTM, states: "After the wheel chocks are in place: Parking Brake - Release".
On all landings there will be some heat build up and long taxi routes exacerbate that heat build up. It is quite possible to arrive at the terminal with brakes so hot that you cannot put your hand on the them without risk of heat injury. Indeed, in a Boeing 737 Operators Symposium of many years ago, Boeing reported that if the hand cannot be held on the brakes then by defintion the brakes were hot and appropriate precautions should be taken.
Most 737's do not have brake temperature gauges in the cockpit. The pilot has no accurate way of assessing the temperature of the brakes after landing or arrival at the terminal. Of course if he suspects the brakes are hot he might instruct the first officer (rank has its priviliges!) to go down the stairs to put his had on the brakes and check their temperature. . Fat chance of that happening in todays democracy...
Recently, an Australian operator changed its shut down procedure from the Boeing recommendation of brakes release after wheel chocks in place, to a revised policy of leaving the parking brake on after chocks in place. Clearly this is a significant change from over 40 years of 737 Boeing recommended practice. The Boeing published FCOM is unchanged however.
The apparent reason given by the operator for this brakes on policy is to "standardise" with other types of aircraft in it's fleet.
Hot brakes in the vicinity of ground personnel and equipment are potentially dangerous. That fact is well publicised in flight safety material. A casual observer might wonder why the pressing need to change a long established Boeing procedure (brakes release when wheel chocks in place) for the seemingly illogical reason to "standardise" with other fleet types. It's got this scribe tossed...